khushikhanera

One morning in Kanniyakumari, 12-year-old Rashmika was cycling to school excited for science class. The teacher asked students to share interesting observations.

Rashmika noticed that the coconut tree shadows were long in the morning but shorter in the afternoon. She thought this happened because the Sun moved across the sky. But she also remembered learning that the Earth moves around the Sun, so she wondered: Does the Sun move, or does the Earth move?

Let’s explore how the Earth, Moon, and Sun work together to create day and night, seasons, and amazing events like eclipses!

Rotation of the Earth

The Earth’s rotation is the spinning motion on its axis, which is an imaginary line passing through the North and South Poles. 

This rotation takes approximately 24 hours to complete one full turn, causing the cycle of day and night.Direction of Rotation

• When viewed from above the North Pole, the Earth rotates counterclockwise, from west to east.
• This rotation makes the Sun appear to rise in the east, move across the sky, and set in the west, although the Sun itself remains stationary relative to the Earth.

Day and Night Cycle

• The Earth’s rotation causes one side to face the Sun, experiencing daytime.
• The opposite side faces away from the Sun, experiencing nighttime or darkness.
• For example, in India, the eastern regions see the sunrise first because they face the Sun earlier during the Earth’s rotation.

Apparent Motion of Celestial Objects

• The Earth’s rotation makes the Sun, Moon, and stars appear to move across the sky from east to west.
• The Pole Star (Dhruva Tara) appears almost stationary because the Earth’s axis points close to it.
• Other stars, such as those in the Big Dipper (Saptarishi), seem to circle around the Pole Star.

Fascinating Fact:

Foucault Pendulum demonstrates Earth’s rotation physically using a swinging pendulum that changes its plane of oscillation over time.

Revolution of the Earth

The Earth’s revolution is its movement around the Sun along a nearly circular path called an orbit. It takes about 365 days and 6 hours to complete one revolution, which defines a year.

Changing View of the Night Sky

• As the Earth revolves around the Sun, it faces different directions in space throughout the year.
• This causes the portion of the night sky visible after sunset to change gradually.
• Different constellations and stars appear at different times during the year.
• This changing view of the night sky helps us observe various celestial patterns and seasons.
• It also allows astronomers to track the movement of stars and planets across the sky.

Fascinating Fact

Astrophotographers use long-exposure photography, keeping the camera’s shutter open for an extended time. This technique captures the apparent motion of stars as curved arcs called star trails, showing how stars seem to move in circular paths due to Earth’s rotation.

Seasons on the Earth

• The Earth’s axis is tilted at an angle relative to its orbit around the Sun.
• This tilt, along with the Earth’s spherical shape, causes the different seasons we experience.
• Around June 21, the Northern Hemisphere tilts toward the Sun, receiving more direct and intense sunlight.
• During this time, the Sun stays above the horizon for more than 12 hours, resulting in summer in the Northern Hemisphere.
• At the same time, the Southern Hemisphere tilts away from the Sun, getting less direct sunlight and shorter daylight hours, causing winter there.
• Around December 22, the situation reverses: the Northern Hemisphere tilts away from the Sun, leading to winter, while the Southern Hemisphere tilts toward the Sun, enjoying summer.
• The difference in the Sun’s intensity and the length of the day during these times causes the changes in temperature and weather that define the seasons.

(a) More intense sunlight in the Northern Hemisphere and less intense sunlight in the Southern Hemisphere in June (b) The opposite situation happens in December

In the Northern Hemisphere (a) Longer daytime in June(b) Shorter daytime in December

Solstices and Equinoxes

• In the Northern Hemisphere, the longest day of the year, called the summer solstice, occurs around June 21.
• The shortest day, known as the winter solstice, happens around December 22.
• On March 21 (the spring equinox) and September 23 (the autumn equinox), day and night are equal, each lasting about 12 hours.
• At the equator, days and nights remain almost equal (12 hours each) throughout the year, with very little seasonal change.

Fascinating Facts 

The polar regions experience extreme daylight conditions — either six months of continuous daylight (known as the Midnight Sun) or six months of darkness.
At the North Pole, the Sun rises on March 21 and stays above the horizon for six months, setting on September 22. The South Pole experiences the opposite—six months of darkness followed by six months of daylight. 

Misconceptions About Seasons

• Seasons are not caused by the Earth being closer to the Sun when tilted toward it or by an oval orbit with significant distance variations. 
• The Earth is actually closest to the Sun in January, but the tilt and spherical shape are the primary reasons for seasonal changes.

Eclipses

Eclipses occur when one celestial body blocks light from another, casting a shadow. The Moon’s position relative to the Earth and Sun causes solar and lunar eclipses.

Solar Eclipse

• A solar eclipse occurs when the Moon comes between the Earth and the Sun, blocking sunlight from reaching certain parts of the Earth.
• Although the Moon is much smaller than the Sun, their apparent sizes in the sky are similar because the Moon is much closer to Earth.
• This alignment allows the Moon to fully or partially cover the Sun.

Types of Solar Eclipses

1. Total Solar Eclipse:

• The Moon completely blocks the Sun, casting a shadow on a small area of the Earth.
• Observers in this area experience total darkness during the day. The sky becomes dark, and the Sun is completely obscured for a few minutes.

Total solar eclipse

A ‘diamond ring’ seen after a total solar eclipse, just when the Moon starts to move away

2. Partial Solar Eclipse:

• The Moon only partially covers the Sun, and observers in the affected area see a part of the Sun obscured by the Moon.

Partial Solar Eclipse 

Safe Viewing of Solar Eclipses: 

• Direct viewing of a solar eclipse is dangerous because the Sun’s intense light can cause permanent blindness, even when the Sun is only partially covered by the Moon.
• Safe methods for viewing a solar eclipse include:
  1. Using specialized solar goggles designed for eclipse viewing.
  2. Attending organized viewing events where safety measures are in place.
  3. Projecting the Sun’s image using a mirror to watch the eclipse safely.
• Superstitions about eclipses, like avoiding eating or going outside, stemmed from historical fears but are no longer necessary with our modern understanding of eclipses.

Duration of the Solar Eclipse

• The total solar eclipse is visible only for a few minutes because of the Earth’s rotation and the Moon’s motion in its orbit. 
• As the Moon moves away from the Sun, the eclipse shifts into a partial solar eclipse, and daylight begins to return.

Historical Views on Solar Eclipses

• Ancient civilizations feared eclipses because they did not understand the phenomenon. They believed that the Sun, a major source of heat and light, had been momentarily blocked by some cosmic force.
• Many superstitions surrounded solar eclipses, with people refraining from activities like eating, cooking, or leaving the house.
• Today, scientists study solar eclipses because they offer a rare opportunity to observe the Sun’s atmosphere and other phenomena that are not visible during normal circumstances.

Fascinating Facts

The Sanskrit word for eclipse is “grahan.” Ancient Indian astronomical texts, such as the Surya Siddhanta, provided calculations to predict eclipses long before modern astronomy developed. These texts use poetic shlokas to describe the phenomenon.

Dive Deeper – Why Planets Can’t Block the Sun? 

• Mercury and Venus are closer to the Sun than Earth, but they are still too small and far away to block the Sun significantly.
• Transit of Venus is a rare event that occurs when Venus moves directly between Earth and the Sun.
• During the Transit of Venus, Venus appears as a tiny black dot crossing the face of the Sun.
• This event is rare because Venus has to align perfectly with the Sun and Earth, which doesn’t happen often.
• Although Venus is much smaller than the Sun, it’s visible as a small dot when it crosses in front of the Sun.

Lunar Eclipse

A lunar eclipse occurs when the Earth comes between the Sun and the Moon, blocking the sunlight from reaching the Moon. This results in the Earth’s shadow falling on the Moon. The event is only visible during a full moon, when the Earth is directly aligned between the Sun and the Moon.

Types of Lunar Eclipses

1. Total Lunar Eclipse:

• In a total lunar eclipse, the Moon is completely covered by the Earth’s shadow.
• During this event, the Moon takes on a dark red color, which is often referred to as a “blood moon.” The reddish hue is caused by the Earth’s atmosphere scattering sunlight, which then reaches the Moon.
• The Moon remains red until it moves out of the Earth’s shadow.

2. Partial Lunar Eclipse:

• In a partial lunar eclipse, only part of the Moon enters the Earth’s shadow. The rest of the Moon remains visible and appears unaffected.
• This creates a distinctive shape where part of the Moon looks darker, and the rest remains bright

Viewing a Lunar Eclipse

• Unlike a solar eclipse, a lunar eclipse is safe to watch with the naked eye. Since the Moon is not as bright as the Sun, the Earth’s shadow provides a dimmer, more diffuse light.
• It is also possible to observe both total and partial lunar eclipses from the Earth without special protection, making them easier for amateur astronomers and enthusiasts to view.

Fascinating Facts

The Kodaikanal Solar Observatory in southern India, established in 1899, has been providing valuable data about the Sun for over 100 years. It is operated by the Indian Institute of Astrophysics (IIA), Bengaluru. The observatory has contributed significantly to solar studies.

Know a Scientist: M.K. Vainu Bappu

• M.K. Vainu Bappu is regarded as the father of modern Indian astronomy. He led efforts to set up many astronomical instruments and telescopes in India, such as those at Manora Peak (Uttarakhand) and Kavalur (Tamil Nadu).
• The observatory at Kavalur has been named after him in recognition of his contributions.
• Bappu made significant discoveries, including a comet, and was known for his studies on stars and solar eclipses. He traveled worldwide to study solar eclipses, contributing greatly to modern astronomical research in India.

Points to Remember

• In the 19th century, scientist Leon Foucault used a long pendulum, known as the Foucault pendulum, to demonstrate the Earth’s rotation. A 22-meter Foucault pendulum is installed in the Constitution Hall of India’s new Parliament building in New Delhi, symbolizing India’s connection to the cosmos.
• Ancient Indian astronomer Aryabhata, in his 5th-century text Aryabhatiya, explained the apparent motion of stars due to Earth’s rotation, comparing it to a person on a moving boat seeing stationary objects move backward. He estimated the Earth’s rotation period as 23 hours, 56 minutes, and 4.1 seconds, remarkably close to the modern value.
• Astrophotographers capture star trails—arcs of star movement in long-exposure photographs—showing the apparent motion of stars due to Earth’s rotation.
• The Bhil and Pawara communities in India’s Tapi Valley used the appearance of specific star patterns to predict monsoon rains, demonstrating traditional knowledge of celestial movements.
• At the North Pole, the Sun rises on March 21 (spring equinox) and remains visible for six months until setting on September 22. The South Pole experiences the opposite, with six months of darkness followed by six months of daylight.
• Ancient Indian texts like the Surya Siddhanta, written in Sanskrit shlokas, provided calculations to predict eclipses, reflecting advanced astronomical knowledge.
• The Kodaikanal Solar Observatory, established in 1899 in Tamil Nadu’s Palani hills, has collected over 100 years of solar data, operated by the Indian Institute of Astrophysics, Bengaluru.

Difficult Words

• Rotation: The spinning motion of an object, like the Earth, around its own axis, completing one turn in about 24 hours.
• Axis of Rotation: An imaginary line passing through the North and South Poles, around which the Earth rotates.
• Revolution: The motion of an object, like the Earth, around another object, such as the Sun, taking about 365 days and 6 hours.
• Orbit: The nearly circular path the Earth follows while revolving around the Sun.
• Solstice: The longest (summer solstice, around June 21) or shortest (winter solstice, around December 22) day in the Northern Hemisphere, due to the Earth’s tilt.
• Equinox: Days when day and night are equal (12 hours each), occurring around March 21 (spring equinox) and September 23 (autumn equinox) in the Northern Hemisphere.
• Solar Eclipse: An event where the Moon blocks sunlight from reaching Earth, causing total or partial darkness in specific areas.
• Lunar Eclipse: An event where the Earth blocks sunlight from reaching the Moon, casting a shadow that makes the Moon appear dark red (total) or partially shadowed (partial).
• Apparent Size: The perceived size of an object in the sky, depending on its actual size and distance from the observer, allowing the Moon to appear as large as the Sun during a solar eclipse.
• Transit of Venus: A rare event where Venus appears as a small black dot crossing the Sun’s face, due to its small apparent size compared to the Sun.

Summary

The Earth’s rotation on its axis, from west to east every 24 hours, causes the day-night cycle and the apparent east-to-west motion of the Sun, Moon, and stars. Its revolution around the Sun, taking about 365 days, leads to changing night sky views and, due to the Earth’s tilted axis and spherical shape, seasonal variations. Solar eclipses occur when the Moon blocks sunlight, creating brief darkness, while lunar eclipses happen when the Earth’s shadow darkens the Moon. These phenomena, safely observed with proper precautions, highlight the dynamic interplay of Earth, Moon, and Sun, studied by ancient and modern scientists alike.

Keshav visits his friend Jatin’s village in the Western Ghats and is amazed by the forests, streams, and especially the glowing dance of fireflies at night. 

He learns from Jatin’s grandparents that fireflies use light to communicate, but their numbers are falling due to light pollution and deforestation.

On the way back to the city, Keshav watches the moonlit hills and wonders — does the Moon produce its own light? 

Or is it just reflected sunlight, like he read in his science book? As he thinks, he notices something interesting: light always travels in a straight line.

Sources of Light

Light enables us to see objects and is produced by various sources, both natural and artificial.

Natural Sources of Light

These include the Sun, stars, fire, lightning, and certain animals (like fireflies) that emit their own light. 

Artificial Sources of Light

Humans have developed several artificial sources over time, beginning with fire created using fuels like oil and wax. 

With technological advancement, electric lights such as bulbs, tube lights, and LED lamps have become common.

Luminous Objects

• These are objects that emit their own light. 
• Examples include the Sun (the primary natural light source), stars, lightning, natural fire, and certain animals like fireflies. 

Non-Luminous Objects 

• These objects do not produce their own light but reflect light from luminous sources. The Moon is a non-luminous object that shines by reflecting sunlight. 
• Other examples include planets like Mars and Venus, and everyday objects like mirrors.

Science and Society

LED lamps are energy-efficient, brighter, and longer-lasting than traditional bulbs. They help reduce electricity bills and are environmentally friendly. Recognizing these benefits, the Indian government actively promotes their use. However, used LED lamps must be properly recycled and not discarded in regular waste.

Try yourself:

What do fireflies use light for?

• A.To communicate
• B.To find food
• C.To attract predators
• D.To navigate

View Solution

​Does Light Travel in a Straight Line?

Light travels in a straight line under normal conditions, a property known as rectilinear propagation.

• When light passes through aligned holes in a straight line, it creates a bright spot on a screen.
• If the holes are misaligned, no spot appears, indicating that light does not bend around obstacles.
• Similarly, a candle flame is visible through a straight pipe but not a bent one, confirming that light follows a straight path.

Viewing candle flame through (a) a straight pipe (b) a bent pipe

Dive Deeper

Light generally travels in a straight line. This can be seen clearly when a laser beam passes through a medium like water. However, under special conditions, light can also bend — a fascinating concept you’ll explore in higher grades.
Caution: Lasers should always be used under teacher supervision. Only low-power laser pointers are safe for basic use. Never point a laser at anyone’s eyes — it can cause serious injury.

Light Through Transparent, Translucent, and Opaque Materials

When light encounters different materials, its behavior depends on the material’s properties.

Transparent Materials

• These allow light to pass through almost completely, enabling clear visibility of objects on the other side. 
• Examples include glass and clear plastic. 

Translucent Materials 

• These allow light to pass through partially, causing diffused or blurred visibility. 
• Examples include tracing paper and frosted glass. 

Opaque Materials

• These block light completely, preventing any light from passing through. 
• Examples include cardboard and thick cloth. 

Shadow Formation

A shadow is a dark region formed when an object blocks light from reaching a surface.

Conditions for Shadow Formation

Shadows require three components: 

• A light source (e.g., Sun or torch), 
• An opaque or partially opaque object to block the light, 
• A screen (e.g., wall, floor, or ground) where the shadow appears.

Types of Shadows

• ​Opaque objects create the darkest, most defined shadows. 
• Translucent objects produce lighter, less distinct shadows.
• Transparent objects may create faint shadows or none at all.

Shadow Characteristics and Observations 

From the above observations we can conclude:

• Shadows are formed when an opaque object blocks light from reaching a surface, which acts as a screen. 
• To observe a shadow, we need a light source, an opaque object, and a screen. 
• The size, shape, and clarity of the shadow depend on the positions of the object, light source, and screen. 
• Changing the colour of the object does not affect the colour of the shadow. 
• Shadows help us understand the presence and shape of objects but do not reveal their colour.

Fascinating Facts 

Shadow play, or shadow puppetry, is an ancient art where flat cut-out puppets are placed between a light source and a screen to create lifelike movements. 

Different Indian regions have unique styles like Charma Bahuli Natya (Maharashtra), Keelu Bomme and Tholu Bommalata (Andhra Pradesh), Togalu Gombeyaata (Karnataka), Ravana Chhaya (Odisha), Tholpavakoothu (Kerala), and Bommalattam (Tamil Nadu). 

These performances entertain and convey important community messages.

Try yourself:

What type of materials allows light to pass through almost completely?

• A.Opaque materials
• B.Transparent materials
• C.Translucent materials
• D.Reflective materials

View Solution

​Reflection of Light

Reflection occurs when light bounces off a surface, changing its direction.

Reflection by Shiny Surface

• Shiny surfaces, such as polished steel plates or plane mirrors, reflect light efficiently, creating bright spots on nearby surfaces. 
• For example, tilting a mirror can direct sunlight onto walls not directly lit. 
• When a light beam hits a plane mirror, it changes direction as a reflected beam, which is why mirrors are used to redirect light in many applications.

Images Formed in a Plane Mirror

A plane mirror is a flat mirror that forms a virtual image of an object placed in front of it.

Image Characteristics

• Same Size: The image is the same size as the object.
• Erect: The image is upright, with the top of the object appearing at the top.
• Laterally Inverted: The left side of the object appears as the right side in the image, and vice versa 
  For example: raising your left arm makes the image raise its right arm). This is called lateral inversion.)
  Another example:  word “AMBULANCE” is written in reverse letters on the front of ambulances so that drivers in vehicles ahead can see the word correctly in their rear-view mirrors. This helps them quickly recognize an approaching ambulance and give way.

Distance Relationship: The image’s distance from the mirror equals the object’s distance from the mirror. Moving closer to the mirror makes the image appear closer, and moving farther makes it appear farther.

Fascinating Fact

The exact time when mirrors were invented is unknown. Early mirrors were made by polishing stone or metal. With the invention of glass mirrors, the traditional art of making metal mirrors faded but still survives in places like Kerala, where the unique Aranmula Kannadi metal mirror has been crafted for centuries.

Pinhole Camera

A pinhole camera is a simple device that uses a tiny hole to form an image on a screen.

Working Principle

Light rays from an object pass through a small hole (pinhole) and form an inverted (upside-down) image on a screen, such as tracing paper. The image shows the object’s colors but is reversed vertically.

Making a pinhole cameraA sliding pinhole camera

• Use two cardboard boxes, one slightly smaller to slide inside the other.Make a small hole in one side of the larger box.
• Cut a square on the opposite side of the smaller box and cover it with thin translucent paper (like tracing paper) to act as a screen.
• Slide the smaller box inside the larger one so the tracing paper is inside.
• Point the pinhole side towards a distant object in bright sunlight.
• Look through the open side, cover your head and camera with a dark cloth to see the image clearly.
• Adjust the smaller box forward or backward until the image appears on the tracing paper.

Try yourself:

What is a characteristic of the image formed by a plane mirror?

• A.It is always larger than the object.
• B.It is the same size as the object.
• C.It is always smaller than the object.
• D.It is always distorted.

View Solution

Characteristics of Image Formed 

• The image formed on the tracing paper is upside down (inverted).
• The images show the colours of the objects on the other side.

Dive Deeper

A pinhole camera forms an upside-down image, while a mirror produces a laterally inverted image that is not upside down. These concepts will be explored further in higher grades.

Making Some Useful Items

The properties of light, such as its straight-line travel and reflection, are used to create practical devices.

Periscope

• A simple periscope is made by placing two plane mirrors inside a Z-shaped box. 
• The reflection from these mirrors allows us to see objects that are not directly visible. 
• Periscopes are commonly used in submarines, tanks, and by soldiers to see outside their bunkers. You can also use one to see over taller people.

A Periscope

Kaleidoscope

• ​A kaleidoscope is made by joining three rectangular plane mirrors in a triangular shape and placing them inside a circular tube. 
• Colored pieces like broken bangles or beads are placed on one end, covered with a transparent sheet and tracing paper. 
• Looking through the open end shows beautiful, ever-changing patterns due to multiple reflections from the three mirrors. 
• Artists often use kaleidoscopes to find inspiration for new designs.

A kaleidoscope

Difficult Words

• Luminous Objects: Objects that emit their own light, such as the Sun, stars, or fireflies.
• Non-Luminous Objects: Objects that do not emit light but reflect light from other sources, such as the Moon or a mirror.
• Rectilinear Propagation: The property of light traveling in a straight line.
• Transparent Materials: Materials that allow light to pass through almost completely, like glass.
• Translucent Materials: Materials that allow light to pass through partially, causing diffused visibility, like tracing paper.
• Opaque Materials: Materials that block light completely, like cardboard.
• Shadow: A dark region formed when an object blocks light from reaching a surface.
• Reflection: The change in direction of light when it bounces off a surface, such as a mirror.
• Lateral Inversion: The left-right reversal of an image in a plane mirror, where the left side appears as the right side.
• Pinhole Camera: A device that uses a tiny hole to form an inverted image of an object on a screen.
• Periscope: A device with two plane mirrors that allows viewing of objects not directly visible, used in submarines and bunkers.
• Kaleidoscope: A device with three mirrors forming symmetrical patterns from reflected objects, used for design inspiration.

All living beings grow and need food for their growth. While animals eat food to grow, what about plants? Have you ever seen plants eating like animals? As animals grow, their size and weight increase, and their bodies change. What changes do you notice in plants as they grow? Food provides nutrients like carbohydrates, fats, proteins, vitamins, and minerals, along with water, all essential for growth.

Let’s explore how plants obtain these nutrients for their growth.

How Do Plants Grow?

Plant growth involves visible changes such as the emergence of new leaves and branches, increased height, and a thicker stem. 

These changes occur as plants obtain essential resources like water, sunlight, and nutrients from their environment.

Role of Sunlight and Water

•  Sunlight provides energy for food production, while water is crucial for nutrient transport and maintaining plant structure. 
• Plants grown with both sunlight and water show better growth, with more leaves, greater height, and vibrant green leaves, compared to those lacking either resource. 
• For instance, a plant without water may wilt or die, and one without sunlight may have pale or yellow leaves due to reduced food production.

Fascinating Fact
The ancient Indian text Vrikshayurveda states, “Trees do not produce fruits and flowers merely because they are planted.” 
This text contains valuable observations about plant growth, soil, and farming methods to improve crop health and yield. 
Based on practical experience and long-term patterns, it systematically guides agricultural practices. 
For example, it describes ways to prepare organic manure using water, barley, and seeds like green gram, black gram, and horse gram.

Try yourself:

What do roots absorb from the soil for plant growth?

• A.Carbon dioxide and chlorophyll
• B.Fruits and flowers
• C.Sunlight and air
• D.Water and minerals

View Solution

How Do Plants Get Food for Their Growth?

Unlike animals, plants produce their own food through a process called photosynthesis, primarily in their leaves. This food, stored as starch (a carbohydrate), provides energy and building blocks for growth.

Leaves: The ‘Food Factories’ of Plants

Leaves are the primary sites for food production due to their broad, flat structure and the presence of chlorophyll, a green pigment that captures sunlight. Chlorophyll enables leaves to convert sunlight, water, and carbon dioxide into food.

Starch Production

• Leaves store food as starch, a carbohydrate. 
• The presence of starch can be confirmed by an iodine test, where a leaf turns blue-black if starch is present. 
• This indicates that the leaf has produced food.

Dive Deeper 

Decolourisation of a leaf in the beginning of testing enables us to easily observe colour change and, thus, the presence of starch.

Fascinating Facts

Some plant leaves look red, violet, or brown because they have more colored pigments than green chlorophyll, which hides the green color. Some of these pigments also assist in photosynthesis. You can perform an iodine test on such leaves to detect starch, which shows that photosynthesis has occurred.

Chlorophyll

• Leaves have green and non-green patches due to the presence or absence of chlorophyll.
• Starch is produced only in the green parts of the leaf where chlorophyll is present.
• Non-green patches usually lack sufficient chlorophyll and do not produce detectable starch.
• The presence of starch can be confirmed by performing an iodine test, where starch turns blue-black.
• Leaves exposed to sunlight produce starch, while leaves kept in darkness do not, even if they have green patches.
• This shows that chlorophyll, in the presence of sunlight, is essential for the preparation of starch in plants.
• Because leaves produce food through this process, they are often called the “food factories” of plants.

Try yourself:

What is the primary function of chlorophyll in plants?

• A.Captures sunlight for photosynthesis
• B.Stores food as starch
• C.Produces oxygen during respiration
• D.Absorbs water from the soil

View Solution

Role of Air in Food Preparation

Air, specifically carbon dioxide, is a key component in photosynthesis. Plants take in carbon dioxide from the air to produce food.

Carbon Dioxide Requirement

• Carbon dioxide from the air is essential for plants to prepare food (starch).
• When a leaf is kept in an environment without carbon dioxide, starch is not produced in that part.
• This shows that carbon dioxide is a key ingredient required for photosynthesis.

Oxygen Release During Photosynthesis

• During photosynthesis, plants release oxygen gas.
• Oxygen production is evident when plants are exposed to sunlight, and bubbles of oxygen can be observed.
• The release of oxygen confirms that photosynthesis happens only in the presence of sunlight.

Activity showing the release of oxygen during photosynthesis

Photosynthesis: In a Nutshell

Photosynthesis is the process by which plants use sunlight, chlorophyll, carbon dioxide, and water to produce glucose (a simple carbohydrate) and oxygen. 

• Glucose serves as an immediate energy source and can be converted into starch for storage. 
• The word equation for photosynthesis is:
  
• Oxygen is released as a by-product, which is vital for the survival of other living beings. 
• Photosynthesis occurs mainly in leaves but also in other green parts of the plant containing chlorophyll.

Know A Scientist: Rustom Hormusji Dastur

Many scientists worldwide have contributed to understanding photosynthesis. In India, Rustom Hormusji Dastur (1896–1961) was a notable plant scientist who studied this process. He served as the head of the Botany Department at the Royal Institute of Science, Bombay (now the Institute of Science, Mumbai) from 1921 to 1935. Dastur researched how factors like water availability, temperature, and light color affect photosynthesis, highlighting their importance in the process.

How Do Leaves Exchange Gases During Photosynthesis?

Leaves have tiny pores called stomata on their surface, which facilitate the exchange of gases during photosynthesis and respiration.

• ​These pores allow carbon dioxide to enter the leaf for photosynthesis and oxygen to exit as a by-product. 
• Stomata also play a role in respiration by allowing oxygen intake and carbon dioxide release.

Stomata

Transport in Plants

Plants have a transport system to move water, minerals, and food to different parts, ensuring growth and survival. 
Roots absorb water and minerals from the soil, which are transported to other parts of the plant through specialized tissues called Xylem.

• All living beings need water to grow, and plants use water for photosynthesis.
• Water and minerals from the soil are absorbed by the roots of plants.
• Minerals are essential nutrients for plant growth.
• Water and minerals travel from the roots to other parts of the plant through a tissue called xylem.
• The xylem consists of thin tube-like structures in stems, branches, and leaves that carry water and dissolved minerals upward.
• The movement of water and minerals can be demonstrated by placing plant twigs in colored water, where the color travels up the stem and into leaves and flowers.
• Water transported through the xylem supports various functions in the plant.

Transport of Food

Food produced in the leaves (glucose or starch) is distributed to non-green parts of the plant, such as roots, stems, and fruits, for growth and storage, through specialized tissue called phloem 

• Leaves are the primary site of photosynthesis, where food is prepared.
•  The phloem is vascular tissue that transports food from the leaves to other parts of the plant. 
• This ensures that areas not involved in photosynthesis, like roots and developing fruits, receive the energy and nutrients they need.
• Some of this transported food is stored in other parts of the plant, such as seeds and roots.

Try yourself:

What is the function of stomata in leaves?

• A.Produce glucose
• B.Exchange gases
• C.Transport water
• D.Store starch

View Solution

Do Plants Respire?

Plants, like animals, respire to produce energy for growth and other functions. Respiration occurs in all parts of the plant, whether green or non-green.

• During respiration, plants break down glucose using oxygen, releasing carbon dioxide, water, and energy. The word equation for respiration is:
  Glucose + Oxygen → Carbon dioxide + Water + Energy
• The energy released supports processes like growth, cell repair, and nutrient transport.
• An experiment with soaked moong bean seeds shows that carbon dioxide is released during respiration, which turns lime water milky.
• This carbon dioxide comes from the seeds respiring inside the flask.
• The energy produced in respiration is used by plants for growth and development.
• All parts of the plant, whether green or non-green, carry out respiration.
• Plants have distinct processes for synthesizing food (photosynthesis), transporting food, and using it to produce energy (respiration).

Respiration in Plants 

Terms to Remember

• Photosynthesis: The process by which plants use sunlight, chlorophyll, carbon dioxide, and water to produce glucose and oxygen.
• Chlorophyll: A green pigment in leaves that captures sunlight for photosynthesis.
• Stomata: Tiny pores on leaf surfaces that allow gas exchange (carbon dioxide in, oxygen out) during photosynthesis and respiration.
• Xylem: Vascular tissue that transports water and minerals from roots to other parts of the plant.
• Phloem: Vascular tissue that transports food (glucose or starch) from leaves to other parts of the plant.
• Respiration: The process by which plants break down glucose using oxygen to release energy, carbon dioxide, and water.
• Glucose: A simple carbohydrate produced during photosynthesis, used as an energy source or stored as starch.
• Starch: A carbohydrate stored in plants, produced from glucose during photosynthesis.

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• Life processes like nutrition, respiration, excretion, and reproduction are essential for the survival of all living beings.
• Animals eat different types of food based on their needs:
  1. Bees and sunbirds suck nectar from flowers.
  2. Human and animal infants feed on mother’s milk.
  3. Snakes swallow their prey whole.
  4. Some aquatic animals filter tiny food particles from water.
• Animals obtain energy from food to carry out life processes.
• Food contains complex components like carbohydrates, proteins, and fats, which must be broken down into simpler forms to be used by the body.
• This breakdown happens in a long tube called the alimentary canal, which starts at the mouth and ends at the anus.Human Digestive System
• Digestive juices secreted at various points in the alimentary canal help break down food.
• The simpler food is absorbed and transported to different body parts for various functions.

Nutrition in Animals

Complex food components must be broken down into simpler forms before the body can use them. This breakdown process is called digestion. Digestion occurs in different ways across animals depending on their type and structure.

To understand digestion better, we first study how it happens in humans.

Digestion in Human Beings

Digestion is the process of breaking down complex food into simpler substances. It occurs in the alimentary canal, a long tube from the mouth to the anus.

Beginning with the mouth cavity

• Digestion begins in the mouth, where food enters and is broken down into smaller pieces by teeth through crushing and chewing. This is called mechanical digestion.
• When you think of your favorite food, your mouth produces more saliva, which moistens the food.
• Saliva contains digestive juices that start breaking down starch (a carbohydrate) into sugar, which is why starchy foods like chapati or rice taste sweet when chewed for some time.

Science and Society

A healthy mouth requires good oral hygiene. We should brush our teeth and clean our tongue twice a day, and rinse our mouth with water after each meal to prevent tooth decay and bad smell in the mouth. Find out the ways our elders were maintaing oral hygiene. 

 Activity to Investigate Saliva’s Role 

Aim: To observe the effect of saliva on the breakdown of starch in boiled rice.

Materials Needed: Two test tubes ,Teaspoon, Boiled rice , Water and Iodine solution 

Procedure:

•  Label the test tubes as ‘A’ and ‘B’. 
•  Place one teaspoonful of boiled rice in test tube A and a teaspoonful of chewed boiled rice in test tube B. 
•  Add 3–4 mL of water to both test tubes. 
•  Note the initial colour of the rice-water mixture. 
•  Add 3–4 drops of iodine solution to each test tube and mix the contents. 
•  Observe the colour changes in both test tubes. 

Observations:

•  In test tube A, the boiled rice may turn blue-black, indicating the presence of starch. 
•  In test tube B, the chewed rice may not change colour or show a very light blue-black colour, indicating the breakdown of starch into simpler sugars. 

2. Food pipe (Oesophagus): A passage from the mouth to the stomach

• After chewing, saliva moistens the food, making it soft and easy to swallow.
• The tongue mixes the chewed food with saliva and pushes it into the food pipe or oesophagus.
• The oesophagus is a long, flexible tube connecting the mouth to the stomach.
• Food moves down the oesophagus by a wave-like motion called peristalsis, where the walls of the food pipe contract and relax gently.Movement of food in the food pipe
• This movement continues throughout the alimentary canal, pushing food forward for digestion.

3. Stomach

• The walls of the stomach contract and relax to churn the food, mixing it thoroughly.
• The stomach lining secretes digestive juice, acid, and mucus.
• Digestive juice breaks down proteins into simpler components.
• The acid helps in protein digestion and kills harmful bacteria.
• Mucus protects the stomach lining from being damaged by the acid.
• Food is partially digested in the stomach and turned into a semi-liquid mass, ready for the next stage of digestion.

Fascinating Facts 

• In 1822, Alexis St. Martin was accidentally shot in the stomach, leaving a small permanent hole after treatment.
• Dr. William Beaumont, his doctor, used this opening to directly observe digestion in the stomach.
• Beaumont conducted experiments to study how different foods are broken down.
• He also explored how emotions can affect the process of digestion.
• This accidental discovery significantly advanced our understanding of stomach function.

Try yourself:

What begins the process of digestion in humans?

• A.Oesophagus
• B.Anus
• C.Mouth
• D.Stomach

View Solution

4. Small Intestine

• After leaving the stomach, partially digested food enters the small intestine, which is about 6 meters long, making it the longest part of the alimentary canal.Alimentary canal if it is stretched out
• The small intestine receives digestive secretions from:
  1. Its own inner lining.
  2. The liver, which produces bile.
  3. The pancreas, which produces pancreatic juice.
• Bile is mildly basic; it neutralizes stomach acid and breaks fats into tiny droplets, aiding digestion.
• Pancreatic juice is also basic and helps neutralize acid, while breaking down carbohydrates, proteins, and fats.
• Digestive juices from the small intestine further break down fats, proteins, and carbohydrates into simpler forms.
• Nutrients are absorbed into the blood through the inner lining of the small intestine.
• The lining has thousands of finger-like projections called villi, which increase the surface area for efficient absorption.
• Absorbed nutrients provide energy, help growth and repair, and support body functions.

Science and Society 

• Celiac disease is a condition where the body reacts to gluten, a protein found in wheat, barley, and rye.
• This reaction damages the inner lining of the small intestine, impairing nutrient absorption.
• As a result, the small intestine cannot function properly.
• The only way to manage celiac disease is to avoid gluten-containing foods.
• Millets such as jowar, bajra, and ragi are good alternatives because they are naturally gluten-free.

5. Large Intestine

• After nutrient absorption in the small intestine, undigested food moves into the large intestine.
• The large intestine is about 1.5 meters long, shorter but wider than the small intestine.
• Its main function is to absorb water and some salts from the undigested food.
• This process turns the waste into a semi-solid form called stool.
• Stool is stored in the rectum until the body is ready to eliminate it.
• The waste is expelled through the anus in a process called egestion.
• Eating fiber-rich foods like fruits, vegetables, and whole grains helps keep the large intestine healthy and stool easier to pass.

Fascinating Facts

• The large intestine contains helpful bacteria that break down undigested fiber.
• These bacteria produce nutrients important for health.
• Eating fiber-rich and fermented foods like curd and pickles supports a healthy digestion.

Science and Society 

• The ancient Ayurvedic text Charaka Samhita stresses the importance of easily digestible foods.
• Spices like ginger, black pepper, and cumin are used to improve digestion.
• Modern nutrition also highlights eating at proper times, mindful eating, and avoiding overeating to maintain good digestive health.

Digestion in Other Animals

Not all animals digest food like humans. Their digestive systems are adapted to their diets and habitats:

Ruminants 

• These grass-eating animals, partially chew it, and swallow it into their stomachs for partial digestion.
• The food is then brought back to the mouth for thorough chewing, a process called rumination.
• Ruminants spend about 8 hours a day chewing their food.
• After chewing, the food moves down the alimentary canal for further digestion.

Birds

• Birds lack teeth but have a special stomach chamber called the gizzard.
• The gizzard breaks down food by contracting and relaxing, often with the help of small stones (grit) swallowed by birds.
• Animals show variations in their digestive systems to suit different types of food.
• Nutrients from digested food help build and repair the body or are broken down to release energy.
• The process of converting nutrients into energy is called respiration.

Try yourself:

What is the primary function of bile produced by the liver?

• A.To neutralize stomach acids
• B.To produce digestive juices
• C.To break down carbohydrates
• D.To absorb nutrients

View Solution

Respiration in Animals

Respiration is the process by which animals use oxygen to break down nutrients (like glucose) to release energy, producing carbon dioxide and water as by-products. 

Breathing, a physical process, brings oxygen into the body and removes carbon dioxide, while respiration is a chemical process occurring in cells.

Respiration in Humans

• Breathing is the process of inhaling (breathing in) oxygen and exhaling (breathing out) carbon dioxide.
• Breathing is essential for survival; without it, humans cannot live more than a few minutes.
• All living beings, including plants and animals, breathe.
• The respiratory system is the body system responsible for breathing and gas exchange.

Human Respiratory System

• Nostrils: Openings in the nose through which air is inhaled and exhaled.
• Nasal Passages: Small passages after the nostrils lined with tiny hairs and mucus that trap dust and dirt. Breathing through the nose is better than through the mouth because of this filtration.
• Windpipe (Trachea): Tube that carries air from the nasal passages to the lungs and divides into two branches.
• Lungs and Alveoli: The windpipe branches further inside the lungs into finer tubes ending in tiny balloon-like sacs called alveoli, where gas exchange occurs.
• Protection: The lungs are protected by the rib cage, a bony structure surrounding them.

Science and Society

• Although the respiratory system filters out much dust from inhaled air, small infectious particles can still enter the lungs.
• For example, during the COVID-19 pandemic, the SARS-CoV-2 virus affected the respiratory system.
• This virus caused breathing difficulties and serious lung problems in many people.

Activity: Understanding Breathing Mechanism through a Simple Model

Materials Needed: Wide transparent plastic bottle with a lid (bottom removed) ,  Y-shaped hollow tube, Deflated balloons (2), Rubber bands, Clay (for sealing) , Thin rubber sheet ,Large rubber band (for securing rubber sheet).

Procedure

Prepare the Bottle: 

•  Take the wide transparent plastic bottle and remove its bottom. 
•  Make a hole in the lid of the bottle. 

Prepare the Tube and Balloons: 

•  Take the Y-shaped hollow tube. Fix the two deflated balloons to the forked end of the tube and secure them with rubber bands to make them airtight. 

Assemble the Model: 

•  Insert the straight end of the tube tightly through the lid of the bottle and seal the lid with clay to ensure it is airtight. 
•  Attach a thin rubber sheet to the open base of the bottle tightly using a large rubber band. 

Conducting the Experiment:

• Pulling the Rubber Sheet Downward: Pull the rubber sheet from the center of the base downwards. Observe the balloons closely. 
• Releasing the Rubber Sheet: Release the rubber sheet upwards and observe the changes in the balloons. Model of Mechanism of Breathing

Observations:

•  When you pull the rubber sheet downwards, the balloons inflate. 
•  Conversely, when you release the rubber sheet upwards, the balloons deflate. 

Process of Respiration Involves: 

(i) Inhalation

•  When you breathe in (inhale), your chest expands as your ribs move up and outwards. 
•  The diaphragm, which is a dome-shaped muscle located below your lungs, moves downwards during inhalation. 
•  This movement increases the space inside your chest, allowing air to enter your lungs. 

(ii) Exhalation

•  When you breathe out (exhale), your ribs move down and inwards, while the diaphragm moves upwards. 
•  This reduces the space inside your chest and helps to push air out of your lungs. 

 In the activity we discussed before, the balloons represent the lungs, and the rubber sheet represents the diaphragm.

Science and Society

Breathing exercises like Pranayama and Tummo have been practiced for centuries worldwide to boost lung health and mental calmness. Pranayama from India enhances respiration and concentration, while Tummo breathing in cold Ladakh improves lung function and body warmth. Deep breathing combined with chanting is also used in many traditions to promote relaxation and clarity of mind.

Try yourself:

What is the first part of the respiratory system where air enters the body?

• A.Nostrils
• B.Windpipe
• C.Alveoli
• D.Lungs

View Solution

What do we Breathe Out? 

When we exhale, the air we breathe out contains more carbon dioxide compared to the air we inhale. 

This can be demonstrated through an experiment with lime water. 

• Lime water is a clear solution that turns milky when it reacts with carbon dioxide.
• Two test tubes with equal amounts of fresh lime water are taken.(a) Air is passed into lime water with a pichkari/syringe (b) Air is exhaled into lime water
• Air similar to inhaled air is passed through lime water in test tube A using a syringe.
• Exhaled air is blown through a straw into lime water in test tube B.
• Test tube B (exhaled air) turns milky, showing a reaction with carbon dioxide.
• Test tube A (inhaled air) shows no change and remains clear.
• This indicates exhaled air contains a higher concentration of carbon dioxide than inhaled air.

(ii) Gas Exchange

• Fresh air enters the lungs through breathing and fills tiny sacs called alveoli.
• Alveoli have thin walls surrounded by blood vessels.
• Carbon dioxide from the blood is released into the alveoli to be exhaled.
• Oxygen from the alveoli passes into the blood and is carried to the entire body.
• Oxygen is used to break down glucose from food, releasing energy in a process called respiration.

Gas exchange through alveoli

• Overall Process of Respiration: In cells, oxygen breaks down glucose to release energy, carbon dioxide, and water. 
  Glucose + Oxygen → Carbon dioxide + Water + Energy
• Inhaled air contains about 21% oxygen and 0.04% carbon dioxide.
• Exhaled air contains about 16–17% oxygen and 4–5% carbon dioxide.
• Breathing is a physical process of air intake and release.
• Respiration is a chemical process inside the body that produces energy.
• Both breathing and respiration are vital for survival.

The percentage of oxygen and carbon dioxide in inhaled and exhaled air

Difference Between Breathing and Respiration

 Role of the Circulatory System

• The circulatory system transports nutrients, oxygen, and waste products in the body.
• It consists of the heart, blood, and blood vessels.
• The heart pumps blood through vessels to deliver oxygen and nutrients and remove wastes.

Science and Society

• Smoking damages the lungs and increases the risk of lung cancer and other respiratory diseases.
• It causes persistent coughing and makes the body prone to frequent infections.
• Smoking releases toxic chemicals into the air, harming others through passive smoking.
• Children, pregnant women, and the elderly are especially vulnerable to passive smoking.
• Avoiding smoking protects both personal health and the health of those around us.

Respiration in Other Animals

Different animals live in different habitats and have different breathing mechanisms suited to their environments.

Animals have adapted respiratory systems based on their habitats:

• Lungs: Animals like birds, elephants, lions, cows, lizards, and snakes breathe through lungs, though lung structures vary.
• Gills: Aquatic animals like fish use gills, which extract dissolved oxygen from water and release carbon dioxide.

Breathing body parts in a fish

• Skin and Gills: Amphibians like frogs use gills as tadpoles, lungs as adults on land, and moist skin for gas exchange in water.
• Moist Skin: Earthworms breathe through their moist skin, allowing oxygen and carbon dioxide exchange.

Try yourself:

What happens to lime water when exhaled air is passed through it?

• A.It turns milky
• B.It remains clear
• C.It changes color
• D.It bubbles

View Solution

Terms to Remember

• Alimentary Canal: The long tube in the digestive system from the mouth to the anus where food is digested and absorbed.
• Mechanical Digestion: The physical breakdown of food into smaller pieces, such as by chewing.
• Peristalsis: The wave-like muscle contractions that move food through the digestive system.
• Villi: Finger-like projections in the small intestine that increase surface area for nutrient absorption.
• Egestion: The process of expelling undigested waste (stool) through the anus.
• Ruminants: Animals like cows that partially digest food, regurgitate it, and chew it again.
• Gizzard: A muscular chamber in birds that grinds food, often with swallowed stones.
• Alveoli: Tiny air sacs in the lungs where oxygen and carbon dioxide are exchanged.
• Diaphragm: A dome-shaped muscle below the lungs that aids breathing by moving up and down.
• Respiration: The chemical process in cells where oxygen breaks down glucose to release energy.
• Circulatory System: The system (heart, blood, blood vessels) that transports nutrients, oxygen, and waste.

Prerna and her sister were watching a sports channel. Prerna loved running and was the fastest girl in her district’s 100 metre sprint. She was now training for the state level and dreamed of running for India.
Watching old Olympic races, Prerna was amazed at how exact the timing was, even when runners finished together. At school, they used a stopwatch to time races.

Her mother wore a watch, her sister checked time on a phone, and her uncle had a Braille and a talking watch. There was also a big clock at school.
Prerna wondered how people in the past told time without these devices.

Let’s explore the history of time and learn more about how people have measured it over the ages.

Measurement of Time

Long ago, humans became interested in keeping track of time. They noticed many natural events happened in regular cycles—like the Sun rising and setting, the phases of the Moon, and changing seasons. These cycles helped people create calendars. A day was defined by the cycle of the Sun rising and setting.

Next, people wanted to know the time during the day. Since there were no clocks or watches, they invented devices to measure smaller parts of the day, such as:

To measure time within a day, they invented devices such as:

• Sundials: These used the shadow cast by the Sun’s light on an object to show the time of day. As the Sun moved, the shadow’s position changed.
• Water Clocks: These measured time using water flow. One type had water flowing out of a marked vessel, while another used a bowl with a hole that sank when filled with water.
• Hourglasses: These used sand flowing from one bulb to another to measure time.
• Candle Clocks: These were candles with markings that showed time as they burned down.

Fascinating Facts 

The world’s largest stone sundial, called the Samrat Yantra, was built about 300 years ago at the Jantar Mantar in Jaipur, Rajasthan. This site is a UNESCO World Heritage place with many ancient astronomical instruments.

The Samrat Yantra stands 27 metres tall. Its shadow moves very slowly—about 1 millimetre every second—and falls on a scale that can measure time as precisely as every 2 seconds. Like all sundials, it shows local solar time, so a small correction is needed to convert this to Indian Standard Time.

Should we make a simple water clock? Let’s us Construct 

 A water clock is a device that measures time by the flow of water from one container to another. It works on the principle of constant water flow, where the time taken for a certain amount of water to flow from the upper part to the lower part indicates the passage of time. 

Materials Required

•  Used transparent plastic bottle (1/2 litre or larger) with cap 
•  Drawing pin 
•  Water 
•  Ink or food colour (optional) 

Procedure

• Prepare the Bottle: Cut the plastic bottle into two halves roughly in the middle. 
• Make a Hole: Use a drawing pin to make a small hole in the cap of the bottle. 
• Assemble the Clock: Place the upper part of the bottle (with the hole in the cap) upside down on the lower half. 
• Fill with Water: Fill the upper part of the bottle with water. You can add a few drops of ink or colour to make the water level more visible. 
• Start the Clock: The water will start dripping into the lower part of the bottle. Use a watch to mark the water level at one-minute intervals until all the water has dripped down. 

How to Use the Water Clock

• Resetting: Pour the water from the lower part back into the upper part. 
• Timing: Watch the water drip into the lower part and mark each time it reaches a level you previously marked. Each mark indicates that one more minute has passed. 

Fascinating Facts 

In ancient India, time was measured using shadows and water clocks.

• The earliest mention of measuring time by shadows comes from the Arthashastra by Kautilya (2nd century BCE to 3rd century CE).
• Around 530 CE, Varahamihira gave a precise way to calculate time using the shadow of a vertical stick.
• Water clocks, where water flowed out of a vessel, were also described in ancient texts like the Arthashastra and Sardulakarnavadana.
• These early water clocks were not very accurate because the flow of water slowed as the water level dropped.
• To solve this, the sinking bowl water clock (called Ghatika-yantra) was developed and mentioned by Aryabhata and other astronomical texts.
• The Ghatika-yantra was used continuously in Buddhist monasteries, royal palaces, and town squares. When the bowl sank, time was announced by drums, conch shells, or gongs.
• Although pendulum clocks replaced the Ghatika-yantra by the late 19th century, it was still used in religious places for rituals.

As human civilization advanced and long-distance travel became common, measuring time precisely became very important. This led to the development of mechanical clocks driven by weights, gears, and springs starting from the 14th century.

The invention of the pendulum clock in the 17th century was a major breakthrough, significantly improving the accuracy of mechanical timekeeping.

Know a Scientist: Galileo Galilei

The pendulum clock was invented in 1656 and patented in 1657 by Christiaan Huygens (1629–1695). He was inspired by the earlier work of Galileo Galilei(1564–1642).Huygens’ Pendulum clock

Galileo noticed a lamp swinging back and forth in a church. Using his own pulse to measure time, he found that the lamp took the same amount of time for each swing. After testing different pendulums, Galileo discovered that the time for one complete swing stayed the same for a pendulum of a fixed length.

This discovery helped Huygens create the accurate pendulum clock.

Try yourself:What device measures time using the flow of water?

• A.Candle Clock
• B.Hourglass
• C.Sundial
• D.Water Clock

View Solution

The Simple Pendulum

• A simple pendulum consists of a small metal ball, known as the bob, which is suspended from a rigid support by a long thread.A simple pendulum
• When the bob is at rest, it is in the mean position. If the bob is moved slightly to one side and released, it begins to swing back and forth in an oscillatory motion.
• This motion is periodic because it repeats the same path after a fixed interval of time.
• One complete oscillation of the pendulum occurs when the bob, starting from the mean position O, moves to the extreme position A, changes direction, moves to another extreme position B, changes direction again, and returns to O.
• Alternatively, the pendulum also completes one oscillation when the bob moves from one extreme position A to another extreme position B and then comes back to A.
• The time taken for the pendulum to complete one oscillation is called its time period.

Experiment to Measure the Time Period of a Pendulum

Materials Needed:

• A piece of string about 150 cm long
• A heavy metal ball or stone (bob)
• A stopwatch or watch to measure time
• A ruler to measure length

Procedure:

1. Tie the bob to one end of the string.
2. Fix the other end of the string to a rigid support so that the length between the support and the bob is about 100 cm.
3. Let the bob come to rest in its mean position. Your pendulum is now ready.
4. Gently hold the bob, pull it slightly to one side, and release it without pushing. Make sure the string is taut when you release it.
5. Observe the pendulum oscillating back and forth.
6. Using the watch, measure the time taken for the pendulum to complete 10 oscillations.
7. Record the time in a table. Repeat this measurement 3 to 4 times for accuracy.
8. Calculate the time period by dividing the total time for 10 oscillations by 10.

Observations

Length of the string = 100 cm 

•  The time period of the pendulum is nearly consistent with each measurement. 
•  This indicates that the time taken for one complete oscillation of the pendulum remains fairly constant. 

Conclusion: The pendulum’s time period stays nearly constant because its length and gravity remain unchanged. This makes its swings stable and reliable for measuring time.

Think Like A Scientist 

When experimenting with pendulums using a watch, the following questions can be investigated:

• How does the length of the pendulum affect its time period?
• Do pendulums of different lengths have different time periods?
• Does the mass of the bob affect the time period?

To test these questions:

• Use the same bob and measure the time period of pendulums with two or three different lengths.
• Observe whether the time period changes with length and record the results.
• Keep the pendulum length fixed and test with bobs of different masses to check if mass influences the time period.

Conclusion:

The time period of a simple pendulum depends on its length but not on the mass of the bob. At a given location, all pendulums of the same length have the same time period.

 All clocks, whether ancient or modern, rely on a process that repeats continuously to mark equal intervals of time. 

Dive Deeper

Modern clocks measure time using repeating movements, but instead of pendulums, they use tiny vibrations from quartz crystals or atoms. Early pendulum clocks could lose or gain 10 seconds every day, but today’s atomic clocks are very accurate and lose only one second in millions of years. Scientists keep working to make clocks even better.

SI Unit of Time

• The standard unit of time in the International System of Units (SI) is the second, represented by the symbol “s.”
• Larger units of time include the minute (symbol: “min”. and the hour (symbol: “h” ).
• The conversions between these units are as follows:

Dive Deeper 

Units of time like second, minute, and hour start with a lowercase letter, unless they begin a sentence. Their symbols — s, min, and h — are always lowercase and singular. Do not put a full stop after the symbol unless it is at the end of a sentence. Always leave a space between the number and the unit when writing time. Also, using “sec” for second or “hrs” for hour is incorrect.

Fascinating Facts

The hole in the bowl of the Ghatika-yantra was designed so it took 24 minutes to fill and sink. This time unit was called a ghatika or ghati. It became the standard way to measure time and was used until the end of the 19th century. A 24-hour day was divided into 60 equal ghatis.

Science and Society: Precision in Time Measurement

 In modern society, measuring very small fractions of a second is crucial in various fields: 

• Sports: Timekeeping devices can record events to one-hundredth or one-thousandth of a second to determine race winners. 
• Medicine: Heart monitors like Electrocardiogram (ECG) machines measure heartbeat variations in milliseconds to identify health issues. 
• Music: Digital recordings capture sound thousands of times per second for smooth playback. 
• Technology: Smartphones and computers process signals in microseconds, enabling fast operation. 

 As clocks become faster and more accurate, they contribute to society in ways that may not be immediately noticeable.

Try yourself:

What is the mean position of a simple pendulum?

• A.The position when the bob is at its highest point
• B.The position when the bob is moving to one side
• C.The position when the bob is at rest
• D.The position when the pendulum is swinging

View Solution

Slow or Fast

• When we say something is moving fast or slow, we compare how far it moves in a certain amount of time.
• For example, in a 100-metre race, all runners start together but soon spread out.Boys running a race on a straight track
• The runner who is ahead at a given moment has covered more distance in the same time and is running faster.
• Therefore, the distance covered in a given time helps decide who is faster or slower.
• We say that the faster runner has a higher speed.

Speed

Speed is the measure of how fast an object is moving. It tells us the distance an object covers in a certain amount of time.

• By comparing the distances moved by two or more objects in a unit time, it can be determined which object is moving faster.
• The unit time can be one second, one minute, or one hour.
• The distance covered by an object in a unit time is called its speed.
• The SI unit of speed is metre per second (m/s), based on the SI units of distance (metre) and time (second). For larger distances and times, speed is often measured in kilometre per hour (km/h). 

Example: Ravi’s school is 5.2 km from his house. It took him 20 minutes to reach his school riding on his bicycle. Calculate the speed of the bicycle in m/s.

Solution:
Speed of the bicycle =  Distance covered/ Time Taken
Speed = 5.2 km/20 min
Convert the units = 5.2 x 1000 m / 20 x 60 s = 5200m/1200s = 4.33 m/s
 Answer: The speed of the bicycle is 4.33 m/s.

Activity: To compare the speeds of different trains based on their timetable information. 

Calculate speed for each train 

Comparison

• Fastest Train: The Superfast Train between Station G and Station H is the fastest, with a speed of 150 km/h.
• Slowest Train: The Passenger Train between Station A and Station B has the slowest speed of 48.19 km/h.

Relationship betwen speed, distance and time

• We know the formula to calculate speed is:
• This formula can be rearranged to find distance if speed and time are known:

• Similarly, to find time if distance and speed are known:

Example 2:
Question: Priya is traveling to a nearby town in a car moving at a speed of 60 km/h. If it takes her 3 hours to reach the town, how far is the town?

Solution:
Distance covered by the car = Speed × Time
= 60 km/h × 3 h
= 180 km

Answer: The town is 180 km away.

Example 3:

Question: A train is traveling at a speed of 80 km/h. How much time will it take to cover a distance of 240 km?

Solution:
Time taken by the train = Distance covered / Speed
= 240 km / 80 km/h
= 3 h

Answer: The train will take 3 hours to cover 240 km.

Average Speed

• The speed calculated by dividing total distance by total time is the average speed.
• An object may not move at the same speed throughout; sometimes it may move slower or faster.
• In this context, the term speed is used to mean average speed.

Science and Society: Measuring Speed and Distance in Vehicles

• Vehicles like scooters, motorbikes, cars, and buses have an instrument called a speedometer.
• A speedometer shows the vehicle’s speed in kilometres per hour (km/h).
• Another instrument called an odometer measures the total distance travelled by the vehicle in kilometres.

Uniform and Non-uniform Linear Motion

Linear Motion

• When an object moves along a straight line, the motion is called linear motion.
• Example: A train moving on a straight track between two stations. 
• This motion can be uniform (at a constant speed) or non-uniform (with changing speed). 

Uniform Linear Motion

• When an object moves along a straight line with a constant (unchanging) speed.
• It covers equal distances in equal intervals of time.
• Example: Car moving from one point to another  at constant speed.

Non-Uniform Linear Motion

• When the speed of an object keeps changing while moving along a straight line.
• It covers unequal distances in equal intervals of time.
• Example: Car moving from A to B and C to D with changing speeds.

Real-life Note: Uniform motion is an ideal concept. In real life, objects rarely move at a constant speed for long durations — hence, we use average speed.

Case Study :  In Table , data are given for the distances travelled by two trains, X and Y, between the time 10:00 AM and 11:00 AM. 

Which of the two trains is in uniform linear motion between 10:00 AM and 11:00 AM? 

 Solution : Train X covers equal distances in equal intervals of time, so it is in uniform linear motion while Train Y is in non-uniform linear motion.

Answer:

• Train X is in uniform linear motion between 10:00 AM and 11:00 AM because it covers equal distances (20 km) in equal time intervals (10 minutes).
• Train Y, on the other hand, is in non-uniform linear motion because the distances it covers in each 10-minute interval are not equal (e.g., 20 km in the first interval, 15 km in the second, and so on).

Thus, Train X moves uniformly, while Train Y does not.

Terms to Remember

• Sundial: A device that uses the Sun’s shadow to show the time of day.
• Water Clock: A device that measures time by the flow of water in or out of a vessel.
• Hourglass: A device that measures time by the flow of sand between two bulbs.
• Candle Clock: A candle with markings that shows time as it burns.
• Pendulum: A weight (bob) hanging from a fixed point that swings back and forth to measure time.
• Time Period: The time taken for one complete oscillation of a pendulum.
• Second: The SI unit of time, symbolized as “s.”
• Speed: The distance covered by an object in a unit of time, measured in m/s or km/h.
• Speedometer: An instrument in vehicles that shows speed in km/h.
• Odometer: An instrument in vehicles that measures distance traveled in km.
• Linear Motion: Motion along a straight line.
• Uniform Linear Motion: Motion along a straight line at a constant speed, covering equal distances in equal times.
• Non-uniform Linear Motion: Motion along a straight line with changing speed, covering unequal distances in equal times.
• Average Speed: The total distance covered divided by the total time taken, used when speed varies.
• Ghatika-yantra: A sinking bowl water clock used in ancient India to measure time in ghatis.
• Ghati: A time unit of 24 minutes, measured by the Ghatika-yantra.

Pema and her brother Palden live in Gangtok. One cold winter evening, they sit near a fireplace. Palden talks about his trip to Kerala during winter. He says Kerala is warmer and more humid than Gangtok.

Their grandfather explains, “Kerala is closer to the equator and has a long coastline, so it is warmer and more humid.” Palden adds, “We learned that the Sun is the main source of heat and light, and places near the equator are usually hot.”

While they talk, Pema watches her grandmother cooking thukpa (a traditional Sikkimese dish) in a big metal pan. She asks, “Why are cooking utensils usually made of metal?” Palden answers, “Because metals are good conductors of heat,” which they studied in their science chapter.

Let’s dive into nature of heat and its transfer in nature!

Conduction of Heat

Conduction is the process by which heat is transferred from the hotter part of an object to its colder part through direct contact. In solids, especially metals, particles vibrate when heated and pass this energy to neighboring particles without moving from their positions. This makes conduction an important process in everyday activities like cooking.

How Conduction Works: When one end of a metal object is heated, the particles at that end gain energy and vibrate more. These vibrations are passed to adjacent particles, transferring heat along the object. For example, when a metal pan is heated, the heat travels from the flame to the entire pan, making it hot.

Let us perform an activity to learn why certain materials are good conductors of heat.

Materials Needed: Metal strip (aluminium or iron), Four pins, Candle or spirit lamp, Stand (or two bricks for support), Wax to attach the pins, Heat source

Steps:

• Take a metal strip (15 cm long) and attach four pins to it using wax, spaced about 2 cm apart.
• Secure the strip to a stand or between two bricks.
• Heat the end of the strip away from the stand with a candle or spirit lamp.
• Observe and predict the behaviour of the pins.

Heat transfer in a metal strip

Prediction: You are asked to predict the order in which the pins will fall as the strip is heated.

Observation: The first pin (Pin I), closest to the candle flame, falls first, followed by the other pins in order (II, III, and IV). The reason for the sequential fall of the pins is the process of heat conduction.

Conclusion

• Conduction is the process of heat transfer from the hotter part of a material to the cooler part. As the heat travels along the metal strip, it causes the wax holding each pin to melt, leading to the pin falling.
• The heat is transferred from the hot end (near the flame) to the colder end through the metal particles vibrating and passing on the energy to their neighbours. 
• Metals are good conductors of heat, which is why metal utensils are used for cooking.

Conductors and Insulators of Heat

1. Good Conductors of Heat:

• Metals (e.g., aluminium, iron) allow heat to pass through them easily.
• This is why metal cooking utensils are commonly used.

2. Poor Conductors of Heat (Insulators):

Materials like wood, glass, clay, and porcelain do not allow heat to pass through easily.
For example:

• Tea or coffee cups made of clay or porcelain help in retaining heat longer.
• Woollen fabrics trap air, which is a poor conductor, and help keep us warm.
• The presence of air between layers of clothing (such as woolen clothes or blankets) reduces heat flow and helps keep us warm. 

Air trapped between two thin blankets acts as an insulator

How Air Acts as an Insulator and Its Applications in Daily Life

1. Woollen Fabric and Heat Retention

• Woollen fabric traps air in its tiny pores or gaps.
• Since air is a poor conductor of heat, it reduces the flow of heat from our bodies to the cooler surroundings.
• This trapped air acts as insulation, helping to keep us warm in cold weather.
• Similarly, when multiple layers of clothing trap air between them, the air acts as an insulator and keeps the body warm.

2. Blankets and Trapped Air

• The presence of air between two thin blankets makes them warmer compared to a single thick blanket.
• This is because the air layer between blankets slows down heat loss from our body, making us feel warm and cozy.

3. Insulated Houses: Using Heat Transfer Principles

• Is it possible to build houses that remain comfortable inside despite very hot or cold outside weather?
• Yes, houses in extreme climates use the concept of heat transfer to stay warm in winter and cool in summer.

4. Hollow Bricks and Heat Insulation

• Some houses use hollow bricks for their outer walls.
• Air trapped inside the hollow parts of these bricks acts as a poor conductor of heat.
• This trapped air helps keep the house warm in winters by reducing heat loss and cool in summers by reducing heat gain.
• Thus, hollow bricks help maintain a comfortable indoor temperature by slowing heat transfer through the walls.

Fascinating Facts 

• The upper regions of the Himalayas, such as the Mori block of Uttarkashi in Uttarakhand, experience extremely cold climates and heavy snowfall in winters.
• To keep houses warm in such harsh conditions, people build walls with two wooden layers.
• The space between these wooden layers is filled with cow dung and mud.
• Both wood and mud are poor conductors of heat, meaning they do not allow heat to escape easily.
• This natural insulation prevents heat loss, helping keep the houses warm and cozy during winter.

Try yourself:

What is conduction?

• A.Heat transfer through liquids
• B.Heat transfer through direct contact
• C.Heat transfer through air
• D.Heat transfer through radiation

View Solution

Convection

Convection is the process of heat transfer in liquids and gases, where heated particles move and carry heat with them. This movement creates currents, such as breezes near the sea or the rising of smoke, making convection a key process in nature.

How Convection Works?

• When a liquid or gas is heated, its particles gain energy, become less dense, and rise. 
• Cooler, denser particles then move to take their place, creating a cycle of movement called a convection current. 
• For example, when water is heated in a pan, the warmer water rises, and cooler water sinks, distributing heat.

Convection in Gases: In air, heated air expands, becomes lighter, and rises.

For example: Smoke rises because it is made up of hot gases and tiny solid particles that are released when something burns. 

When these particles are heated, they become lighter than the surrounding air, causing them to rise. 

This is similar to what happens when air is heated; it expands, takes up more space, and becomes less dense, which is why warm air also rises. 

Convection in Liquids: In liquids like water, heated particles rise, and cooler ones sink, creating a circular flow. 

To understand why smoke rises more clearly, let us perform an activity. 

Materials Needed: paper cups, a wooden stick, threads, a burning candle. 

Procedure:

•  Hang the paper cups in an inverted position on the wooden stick using threads. 
•  Place the stick horizontally. 
• Light a candle and place it under one of the cups. 
•  Observe what happens to the cups. 

Observations and Explanation:

•  The cup with the candle underneath it rises because the air inside the cup heats up. 
•  When air is heated, it expands and takes up more space, making it lighter. This causes the cup to rise. 

Hot air rising up

Example of Air Expansion:

•  When a partially inflated balloon is placed in the Sun, the air inside it heats up and expands, causing the balloon to become larger. 

Smoke Rising:

•  When an incense stick is burnt, the smoke produced is a mixture of hot gases and tiny solid particles. 
• Since the smoke is warmer than the surrounding air, it rises. 

Convection in Liquids:

To understand how heat transfer occurs in liquids, we can perform an activity. 

Materials Needed: 500 mL beaker, water, straw, potassium permanganate, candle. 

Procedure:

• Fill the beaker halfway with water.
• Using a straw, place a grain of potassium permanganate at the center of the beaker’s base.
• Place a candle under the center of the beaker’s base.
• Observe the movement of the colored streak in the water.

Observations:

•  As heat is supplied, a streak of color starts moving up from the center and coming down from the sides of the beaker. 

Demonstration of convection in heated water 

Explanation:

•  The water at the bottom of the beaker gets heated, becomes lighter due to expansion, and rises. 
•  The cooler, heavier water from the sides then comes down to take its place. 
•  This creates a continuous cycle until the entire volume of water is heated. 

Conclusion:

•  The movement of the colored streak in the water demonstrates convection, which is how heat transfer occurs in liquids and gases through the movement of particles. 
•  Just like air, water gets heated through convection, where particles move from one place to another, carrying heat with them. 

Land and Sea Breeze

• During the day, the land near the beach heats up faster than the water in the sea. This is because different materials absorb heat at different rates. 
• However, at night, the situation changes: the land cools down faster than the water. 
• This difference in how quickly land and water heat up and cool down is what causes the land and sea breeze. 

Let us check how land and water get heated and cooled by performing an activity.  

Materials Required: Two identical bowls, Soil , Water and Two laboratory thermometers 

Procedure:

1.  On a clear, sunny day, under the supervision of a teacher or an adult, take two identical bowls. 
2.  Fill one bowl halfway with soil and the other bowl halfway with water. 
3.  Fix a laboratory thermometer in each bowl, ensuring that the bulbs are immersed in the soil and water, respectively, and do not touch the bottoms or sides of the bowls. 
4.  Place the set-up in sunlight. 
5.  Observe the rise in temperature of the soil and water over a period of time. 

Observations:

 After 20 minutes, you will find that the temperature of the soil rises more than that of the water. This indicates that the soil heats up faster than water. 

Cooling Experiment:

1.  After letting the soil and water heat up, bring the set-up indoors and allow it to cool for 20 minutes. 
2.  Observe the cooling rates of the soil and water. 

Conclusion:

•  Soil heats up faster than water. 
•  Soil also cools faster than water. 

Sea Breeze

• During the day, when the land heats up quickly, the air above the land also becomes warm and rises. 
• This creates a low-pressure area over the land. 
• Meanwhile, the air above the sea is cooler and denser. 
• To fill the low-pressure area over the land, the cooler air from the sea moves in, creating a sea breeze. 
• This is why people living in coastal areas feel a refreshing breeze coming from the sea during the day. 

Sea Breeze

Land Breeze

• At night, the land cools down faster than the sea. 
• The air above the land becomes cooler and denser, creating a high-pressure area.
• Meanwhile, the air above the sea is still relatively warm and rises, creating a low-pressure area. 
• To balance the pressure, the cooler air from the land moves towards the sea, creating a land breeze. 
• This is why people living near the shore experience a change in wind direction from day to night.

Land Breeze

Try yourself:

What happens to air when it is heated?

• A.It becomes less dense and rises.
• B.It becomes denser and sinks.
• C.It cools down immediately.
• D.It remains the same.

View Solution

Radiation

• Radiation is heat transfer do not need any medium. All objects emit heat this way. 
• Heat transfer happens directly from the a hot object to us through a process called radiation.
• For example, the Sun’s heat reaches Earth by radiation. The Sun’s hot surface (about 6000°C) emits energy waves, some of which warm the Earth. 
• Radiation does not require a medium, which is why we can feel the warmth of the Sun even though space is a vacuum. 

Examples of Heat Transfer in Daily Life ( Combining all the three Processes)

Many everyday examples show conduction, convection, and radiation happening at the same time. 
For example, when water is heated in a pan:

• Heat moves from the flame to the pan by conduction.
• Water inside the pan heats up by convection.
• The warmth we feel around the flame and pan is due to radiation.

Fascinating Facts: The Himalayan Bukhari

In the upper Himalayan region, a traditional room heater called bukhari is used to keep rooms warm in winter. It is an iron stove where wood or charcoal is burned. A long pipe attached at the top acts as a chimney to release smoke. The flat top of the bukhari can also be used for cooking by placing utensils on it.

When the bukhari is used, all three types of heat transfer—conduction, convection, and radiation—work together to warm the room and cook food.

Water Cycle

Water exists in three states in nature

Liquid: in oceans, rivers, lakes

Solid: as snow, glaciers, ice sheets in mountains and polar regions

Gas: as water vapor in the atmosphere

• During summer, snow and ice melt due to the Sun’s radiation, forming rivers that flow into oceans. Fresh snow replenishes the ice in winter.
• Water in oceans, rivers, and lakes evaporates due to the Sun’s heat. Plants also release water vapor through transpiration.
• Water vapor rises, cools, and condenses to form clouds. Clouds cause precipitation (rain, snow, hail).
• This continuous movement of water—evaporation, condensation, precipitation, infiltration, and runoff—is called the water cycleWater cycle

Importance of Water Cycle

The water cycle is the process through which water continuously moves upward as water vapor and downward through precipitation, passing through soil, rocks, and plants before returning to water bodies. This cycle helps to redistribute and replenish water in rivers, lakes, and oceans while conserving the total amount of water on Earth.

Know a Scientist: Varahamihira

Varahamihira was a famous astronomer and mathematician of the 6th century CE from Ujjaini (now Ujjain), Madhya Pradesh. In his work Brihatsamhita, he described methods to predict seasonal rainfall. His predictions were based on observations of cloud formation, wind patterns, the positions of stars and the moon, and other natural phenomena.

Seepage of Water Beneath the Earth’s Surface

Let’s Fisrt  Perform Activity to understand : How does water seep through the surface of the Earth?

•  Take three transparent, used plastic bottles of 1 L capacity. 
•  Cut them in the middle and make a small hole in the cap of each bottle. 
•  Keep them inverted and put some clay in one bottle, sand in the second, and gravel in the third. 
•  Place three identical beakers below each bottle. 
•  Add 200 mL of water to each bottle. 
•  Predict the amount of water flowing out of each bottle. 
•  Collect the water that flows through each bottle for 10 minutes. 
•  Compare the amount of water that comes through each bottle. 

An activity to compare the flow of water through clay, sand and gravel

You may have observed that water flows fastest through gravel, slower through sand, and slowest through clay. This is due to the differences in the particle sizes and the spaces between them:

• Gravel: The spaces between gravel particles are wider, allowing water to pass through quickly.
• Sand: The particles are smaller than gravel, so the spaces are narrower, causing slower water flow.
• Clay: The smallest particles create very tight spaces, restricting water flow the most.

Now let’s learn the theory and definition

• When rainwater or surface water seeps down through the soil and rocks beneath the Earth’s surface, this process is called infiltration.
• The ease with which water infiltrates depends on the size and connectivity of the spaces between soil and rock particles. If these spaces (called pores) are wider, open, and well connected, water seeps through more quickly and easily.
• Once water infiltrates, it moves down and gets stored in the tiny spaces or pores within sediments (loose soil, sand, gravel) and the cracks or openings in rocks below the surface. This stored water is known as groundwater.
• The underground layers of sediments and rocks that hold this water are called aquifers.
• We access groundwater by digging wells or drilling bore wells into these aquifers.
• The depth of groundwater can vary greatly — it might be just a few meters below the surface or hundreds of meters deep, depending on the region.
• Although groundwater is a vital source of water, it is not unlimited.
• Increasing population and their growing water needs have caused excessive extraction of groundwater.
• Urbanization has reduced the natural areas where water can seep into the ground:
  1. Less vegetation cover means fewer plants and trees to help water infiltrate.
  2. More concrete surfaces (roads, buildings) prevent water from soaking into the soil.
• Due to these factors, the rate of groundwater recharge is reduced, leading to groundwater depletion.
• To conserve and replenish groundwater, techniques like:
  1. Rainwater harvesting — collecting and storing rainwater for later use.
  2. Recharge pits — specially made pits that help rainwater seep into the ground.
• These methods help recharge groundwater, supporting the natural water cycle and ensuring a sustainable supply of groundwater for future needs.
• Water scarcity makes life hard, so people find ways to save water.
• In Ladakh, they make ice stupas—artificial ice cones built in winter.
• These ice stupas melt slowly in warmer months, providing water for farming and daily use.

Science and Society: Ice Stupa

• In Ladakh, spring streams often dry up because the sun’s heat is not enough to melt mountain snow quickly.
• During winter, water from mountain streams is sent through underground pipes and sprayed into the cold air.
• The water freezes layer by layer, forming a tall, cone-shaped ice structure called an ice stupa.
• The ice stupa melts slowly in spring, supplying water for farming and other needs throughout summer.Ice Stupa

Try yourself:

What process does the water cycle involve for returning water to the Earth’s surface?

• A.Evaporation
• B.Infiltration
• C.Transpiration
• D.Condensation

View Solution

Terms to Remember

• Conduction: The process of heat transfer through direct contact, where particles pass heat to neighboring particles without moving.
• Convection: The transfer of heat in liquids and gases by the movement of heated, less dense particles.
• Radiation: The transfer of heat through waves, without needing a medium, as seen in heat from the Sun or a fire.
• Insulator: A material (e.g., wood, glass) that does not allow heat to pass through easily, also called a poor conductor.
• Evaporation: The process where water turns into vapor due to heat, especially from the Sun.
• Condensation: The cooling of water vapor to form liquid droplets, creating clouds.
• Precipitation: The release of water from clouds as rain, snow, or hail.
• Transpiration: The release of water vapor by plants into the atmosphere.
• Aquifer: Underground layers of rock or soil that store groundwater.
• Seepage: The process by which water moves through soil or rock into the ground.
• Bukhari: A traditional Himalayan heater that uses wood or charcoal to warm rooms and cook food.
• Ice Stupa: A cone-shaped ice structure in Ladakh that stores water in winter and releases it in spring.

Summary

Heat transfer occurs through three processes: conduction, convection, and radiation. Conduction involves heat moving through solids, like metals, by particle vibrations, making metals ideal for cooking utensils. Convection occurs in liquids and gases, where heated particles move, creating currents like sea and land breezes. Radiation transfers heat without a medium, as seen in the Sun’s heat or warmth from a fire. The water cycle, driven by the Sun’s heat, involves evaporation, condensation, precipitation, and seepage, redistributing and conserving Earth’s water. Groundwater is replenished through seepage, but conservation methods like ice stupas and rainwater harvesting are vital to address scarcity.

The life of a plant starts from a tiny seed. With proper care, it grows into a sapling and then a mature plant, which can produce flowers, fruits, and new seeds. But it takes time for a plant to grow before it can make seeds again.

Similarly, animals and humans also grow and change before they can reproduce. Some animals lay eggs, while others give birth to young ones. These young ones grow slowly and develop over time.

In humans, life moves through different stages: infancy, childhood, adolescence, adulthood, and old age. Between ages 10 to 19, we go through adolescence, a time of fast physical, emotional, and mental growth. During this stage, the body begins to prepare for adulthood and reproduction.

Let’s explore the changes that happen during adolescence and learn how to handle them responsibly.

Growing with Age: The Teenage Years

What is Adolescence?

Adolescence is the transitional stage between childhood and adulthood, starting around age 10 and lasting until age 19, characterized by rapid physical and other changes.

Changes During Adolescence

As children grow into teenagers, their bodies and minds go through many physical and emotional changes. These changes are natural and happen at different times for different people.

Here are some common changes:

1. Increase in Height and Body Shape

• Boys and girls grow taller quickly.
• Boys may gain weight, have broader shoulders, and wider chests.
• Girls may notice breast development and body shape changes.

2. Change in Voice

• Boys’ voices become deeper due to the growing voice box (also called the Adam’s apple).
• Girls may also experience slight changes in their voice.

3. Hair Growth

• Hair begins to grow in the armpits and pubic area in both boys and girls.
• Boys also develop facial hair like a moustache or beard, and sometimes chest or back hair.
• The amount and timing of hair growth vary from person to person.

4. Changes in Skin

• Many teens get pimples or acne because of oily skin during this time.
• Acne is common and happens when oil blocks skin pores.

A Key Point to Remember

• Everyone goes through these changes at their own pace.
• Some may grow faster or slower, and that’s completely normal.
• These changes are part of growing up.

Secondary Sexual Characteristics

Certain changes, like voice deepening in boys, facial and chest hair growth in boys, and breast development in girls, are called secondary sexual characteristics.

• These characteristics distinguish males from females but are not directly involved in reproduction.
• They signal the onset of puberty, the stage where the body prepares for adulthood and reproductive capability.

What is Puberty?

Puberty is the stage when both external and internal changes prepare the body for reproduction. It marks the beginning of adolescence and the path toward becoming an adult.

Try yourself:

What is the age range for adolescence?

• A.10 to 19 years
• B.5 to 10 years
• C.20 to 25 years
• D.1 to 5 years

View Solution

Changes that Indicate Reproductive Capability

Adolescence brings not just visible changes like height or voice, but also important internal changes — especially those related to reproduction.

Menstrual Cycle in Girls:

The menstrual cycle, commonly called “the period,” is a natural process in adolescent girls, marking reproductive maturity.

• It usually occurs every 28–30 days, but it can vary between 21–35 days.
• The time when blood comes out from the body is called menstruation and it lasts for 3 to 7 days.
• Some girls may feel pain or discomfort in the lower belly during this time.
• Menstruation is a normal and healthy sign of reproductive growth.
• It naturally stops between the ages of 45–55, marking the end of a woman’s ability to reproduce.

Menstrual Cycle

Breaking Myths About Menstruation

• In many places, there are wrong beliefs and taboos about periods.
• Some believe girls should stay isolated during menstruation — this is not correct.
• Menstruation is natural and healthy, and there’s no need for shame or fear.
• Understanding it with a scientific outlook helps support women’s health and removes unnecessary taboos.

Emotional and Behavioral Changes in Adolescents

Adolescence is not only a time of physical growth — it’s also a time of strong emotions and changes in behaviour. These changes can feel exciting, confusing, or even overwhelming, but they are a normal part of growing up.

Let’s understand some common emotional changes and how they affect behaviour — along with ways to handle them positively.

Emotional Changes, Behaviour, and Positive Growth

Understanding and Managing Emotions

• Adolescents often feel emotions more strongly than children.
• These emotions can lead to new interests, such as helping others or exploring creative hobbies.
• By understanding your feelings, you can make thoughtful choices and respond to situations in a healthy and balanced way.

Try yourself:

What is the typical duration of menstruation?

• A.3–7 days
• B.1–2 days
• C.10–14 days
• D.5–10 days

View Solution

Making Adolescence a Joyful Experience

Adolescence is a special journey filled with curiosity, energy, and new experiences. To make this stage happy and healthy, it’s important to take care of your body and mind. Good habits, smart choices, and staying positive can make a big difference!

Meeting Nutritional Needs

Adolescence requires a nutritious diet to support rapid growth and development.

Nutrients Needed

• Proteins and Carbohydrates: Essential for growth, strength, and energy, found in foods like milk, millets, and curd.
• Calcium: Supports bone growth, found in milk, cheese, and paneer.
• Iron: Aids blood formation, found in spinach, kidney beans, and dried fruits like raisins.
• Fats, Vitamins, Minerals: Needed in adequate amounts for overall health.
• Health Concerns: Adolescents, especially girls, may face blood-related issues due to iron or vitamin B12 deficiency, which can be managed with a balanced diet.

Science and Society: Iron Deficiency in Adolescents

Many adolescents, especially girls, may suffer from iron deficiency. This can lead to tiredness, weakness, or even anaemia (a blood-related health problem).

To prevent this:

• Eat iron-rich foods like spinach, jaggery, lentils, and dried fruits
• Include vitamin C (like oranges or amla) with iron-rich foods to help absorb iron better
• Government programs like Iron and Folic Acid (IFA) supplementation provide free tablets in many schools

Know a Scientist: Dorothy Hodgkin

Dorothy Hodgkin was a great chemist who studied vitamin B₁₂, which is needed for a healthy body and blood.

• She won the Nobel Prize in Chemistry in 1964.
• Our body can’t make vitamin B₁₂, so we must get it from food like dairy, eggs, and meat.
  Talk to your teacher or family to know more about foods rich in B₁₂!

Personal Hygiene

Along with eating healthy food, personal hygiene is essential during adolescence.

• During this stage, your body goes through many changes.
• Keeping your body clean, especially in the armpits and pubic area, helps prevent infections.

Menstrual Hygiene for Girls

• Girls must take extra care of hygiene during menstruation (periods).
• Use sanitary pads or reusable cloth pads to stay clean and comfortable.
• Dispose of used pads properly: wrap them in newspaper and throw them in a covered dustbin.
• Today, biodegradable sanitary pads are also available, which are safe for the environment.

Breaking the Stigma

Menstruation is natural, and there is no need for shame.
Society must:

• Provide clean toilets and safe spaces in schools and public places.
• Help make sanitary products easily available.
• Talk openly about menstrual hygiene to support girls and women.

Science and Society: Government Support for Menstrual Hygiene

Physical Activities

• Benefits: Regular exercise, games, and sports keep the body and mind fit, build stamina, and boost mood.
• Examples: Activities like running, cycling, or team sports promote overall health.
• Importance: Physical activity supports growth, reduces stress, and enhances well-being during adolescence.

Balanced Social Life

We all live together in society and interact with many people every day. Being polite and respectful helps create a friendly and safe environment for everyone.

Adolescence and Social Interaction

• Adolescence brings new feelings and experiences.
• Teens may feel attracted to their friends and often copy their behavior.
• Many now interact online through social media and messaging apps.

Using Technology Responsibly

• Technology lets us connect, learn, and share information easily.
• It’s important to use social media carefully and responsibly.
• Sometimes, people may use social media carelessly or hurt others without meaning to.
• If you face cyberbullying (mean messages, rumors, sharing private info), don’t be scared — tell your parents or teachers right away.

Dos and don’ts to be followed on social media

Science and Society: Cyberbullying

Cyberbullying means using phones or computers to hurt others by sending mean messages or sharing private info without permission. It is important to stay safe and seek help if you face it.

Avoiding Harmful Substances

During adolescence, some people may try to pressure you to use harmful substances like tobacco, cigarettes, alcohol, or illegal drugs. Because of curiosity and excitement, teens might be tempted to try them.

Why Avoid These Substances?

• They are harmful to your body and mind.
• They are addictive, meaning once you start, you might want to keep using them again and again.
• This addiction is called substance abuse.
• Using these substances can cause serious health problems like lung damage, breathing difficulties, and memory loss.

Condition of the lungs before and after prolonged exposure to bidi/cigarette smoke

Be Strong and Say NO!

• Say NO the very first time and every time someone offers these substances.
• Remember, people who are addicted often started with just one try.
• Choose healthy habits instead.
• If you or someone you know is struggling, talk to trusted adults like parents or teachers for support.
• Counselling and medical help are available and important.

Science and Society: Nasha Mukt Bharat Abhiyaan

• This is a government campaign to raise awareness about avoiding substance abuse.
• It focuses on preventing addiction, especially among children and youth.
• A National De-addiction Helpline (14446) is available for help with drug addiction.

Try yourself:

What is essential for growth, strength, and energy during adolescence?

• A.Vitamins
• B.Proteins and Carbohydrates
• C.Fats
• D.Minerals

View Solution

The ‘Why’ Question for Adolescence

• Adolescence involves many physical and emotional changes.
• These changes occur mainly due to hormones, which are chemical messengers produced in the body.
• Hormones regulate growth, development, and various body functions.
• They are released at specific times based on signals from the brain.
• Some hormones also influence mood and behavior during adolescence.
• Understanding these changes, seeking guidance when needed, and making healthy choices help build a strong foundation for life.

Points to Remember

• Adolescence is the stage from about 10 to 19 years when a child grows into an adult.
• Physical changes include getting taller, gaining weight, and developing features like facial hair in boys and breasts in girls.
• Puberty prepares the body for having babies, and girls start their menstrual cycle (period) every 3–4 weeks.
• Menstruation is natural, and using sanitary pads helps keep girls healthy. Myths about periods are not true.
• Teenagers may have mood swings and feel more sensitive, but these feelings can lead to creativity and kindness.
• Eating healthy foods with protein, calcium, and iron helps growth. Lack of iron or vitamin B12 can cause health problems.
• Good hygiene, especially during periods, is important to avoid infections. Governments provide help with sanitary pads.
• Exercise and sports keep the body strong and the mind happy.
• Use social media carefully and ask adults for help if you face problems like cyberbullying.
• Avoid harmful substances like tobacco, alcohol, and drugs because they harm health and cause addiction.
• Hormones cause many changes in the body and feelings during adolescence.
• Learning about these changes and making good choices helps you grow up healthy and strong.

Difficult Words and Their Meanings

• Adolescence: The stage of life between childhood and adulthood (ages 10–19), marked by rapid physical, emotional, and behavioral changes.
• Puberty: The phase during adolescence when the body develops into an adult capable of reproduction, involving external and internal changes.
• Secondary Sexual Characteristics: Physical traits (e.g., facial hair in boys, breast development in girls) that distinguish males from females but are not directly involved in reproduction.
• Menstrual Cycle: A natural process in girls where blood is discharged (menstruation) every 28–30 days, signaling reproductive health.
• Menstruation: The phase of the menstrual cycle involving blood discharge, lasting 3–7 days, typically starting at puberty and ending by ages 45–55.
• Hormones: Chemicals produced in the body that regulate growth, development, mood, and functions like menstruation or puberty.
• Mood Swings: Sudden changes in emotions, common in adolescents, like feeling happy one moment and low the next.
• Personal Hygiene: Practices like keeping the body clean to prevent infections, especially important during adolescence.
• Menstrual Hygiene: Using sanitary pads or cloth pads during menstruation to stay clean and healthy, with proper disposal to protect the environment.
• Cyberbullying: Harassing others online through messages, rumors, or sharing private information, which adolescents should handle by seeking help.
• Substance Abuse: Regular use of harmful, addictive substances like tobacco, alcohol, or drugs, causing health issues like lung damage or memory loss.
• Addiction: A strong urge to repeatedly use harmful substances, making it hard to stop after starting.