Light Chapter Notes | Physics Class 6 ICSE PDF Download

Introduction

Light is very important for us because it helps us see things around us. Without light, everything would be dark, and we wouldn’t be able to see anything. Light comes from different sources like the sun, bulbs, and candles. It travels in straight lines and helps us understand many things like shadows, eclipses, and how a pinhole camera works. In this chapter, we will learn about where light comes from, how it behaves, and why some objects let light pass through while others don’t.

Sources of Light

• We can see things only when light falls on them and reaches our eyes.
• Light is a type of energy that helps us see things by making them visible.
• Anything that gives light is called a source of light.
• The sun, stars, candles, and bulbs are examples of sources of light.
• The sun is a natural source of light because it is made by nature.
• Without the sun’s light and heat, we cannot imagine life on Earth.
• Stars, lightning, and twinkling stars are also natural sources of light because they are made by nature.
• Bulbs, candles, tube lights, and torches are artificial sources of light because they are made by humans.

Luminous and Non-luminous Objects

• Objects that give their own light are called luminous objects.
• The sun, stars, bulbs, and candles are examples of luminous objects because they give their own light.
• Objects that do not give their own light are called non-luminous objects.
• Chairs, tables, trees, and stones are examples of non-luminous objects.
• Non-luminous objects can be seen only when light falls on them and reflects back to our eyes.

Activity 5.1

Aim: To show that non-luminous objects do not give light.

• Go to a completely dark room and try to see things kept there.
• Now, turn on the light and look at the things in the room.

Observation: You cannot see things like a table or chair in the dark, but you can see them when the light is on.

Conclusion: This shows that non-luminous objects do not give light and can be seen only when light from a luminous object falls on them.

Bio-Luminescence

• The moon does not have its own light; it shines by reflecting the sun’s light.
• Other planets also shine by reflecting light because they do not have their own light.
• Some living things like fireflies, glow-worms, lanternfish, and dinoflagellates can make their own light.
• This light made by living things is called bio-luminescence.
• Bio-luminescence is a natural source of light made by chemical reactions inside the body of these living things.
• Animals use bio-luminescence for different reasons:
  • To see better in the dark.
  • To attract their prey (food).
  • To communicate with others.
  • To hide (camouflage).
  • To escape from predators (enemies).

Scattering of Light

• Air has many gases and tiny dust particles.
• Light always travels in a straight line.
• When light passes through air, it gets blocked by dust particles and gases.
• Some light gets absorbed, and some light gets sent in different directions—this is called scattering of light.
• Scattering of light helps us see objects that are not in the direct path of sunlight.
• We can see objects around us at dusk and dawn because of scattering of sunlight.
• In a dark room, if we light a candle or torch, the dust particles scatter the light in all directions.
• This scattering helps us see things that are not directly in the straight path of the candle or torch.

Transparent, Translucent, and Opaque Materials

• Imagine you are in a dark room with no windows—everything is completely dark, and you cannot see anything.
• If the room has glass windows, sunlight can come in, and you can see things because light passes through the windows.
• Not all materials let light pass through them.

Materials are divided into three types based on how much light they let pass:

Transparent Material

• A transparent material lets light pass through it completely.
• Examples of transparent materials are glass, air, and water.
• You can clearly see things through a fully transparent material.
• Transparent materials are used to make things like showcases, windows, doors, spectacles, binoculars, telescopes, and microscopes.

Translucent Material

• A translucent material lets light pass through it only partially (not fully).
• Examples of translucent materials are wax paper, frosted glass, and colored bottles.
• You cannot see clearly through a translucent material—it looks foggy.
• Translucent materials are used to make things like doors, windowpanes of bathrooms, clinics, offices, and laboratory equipment.

Opaque Material

• An opaque material does not let light pass through it at all.
• Examples of opaque materials are wood, doors, metal, books, and brick walls.
• An opaque material always makes a shadow when light falls on it.
• Opaque materials are used to make things where we do not want light to pass through, like walls and doors.

Activity 5.2

Aim: To find out which objects are transparent, translucent, or opaque.

Materials Required: Objects like a pencil, steel spoon, mirror, glass, notebook, ball, pencil box, plastic bag, wax paper, thick cloth, paper cup, and polythene.

Procedure:

• Take the objects one by one in sunlight.
• Check which objects let all the light pass through them (these are transparent).
• Check which objects let only some light pass through them (these are translucent).
• Check which objects do not let any light pass through them (these are opaque).
• Compare your list with your partner’s list.

Observation: I observed that the glass and polythene are transparent, wax paper and plastic bag are translucent, and the pencil, steel spoon, mirror, notebook, ball, pencil box, thick cloth, and paper cup are opaque. After comparing with my partner, our lists matched, confirming the classifications.

Rays and Beam of Light

Light travels in a straight line, and we call this a ray of light. A ray of light is shown as a straight line with an arrow to show the direction it moves. When many rays of light travel together in the same direction, it is called a beam of light. A torch makes a beam of light.
A beam of light can be:

Parallel Beam

• A parallel beam is when many rays of light move side by side in the same direction.
• The sun makes a parallel beam of light because it is far away.

Convergent Beam

• A convergent beam is when many rays of light come together at one point.
• Light passing through a magnifying lens makes a convergent beam.
• An example of a convergent beam is sunlight passing through a magnifying glass.

Divergent Beam

• A divergent beam is when rays of light come from a small source and spread out in different directions.
• Light from a bulb, an LED, or a candle makes a divergent beam.

Rectilinear Propagation of Light

• Light always moves in a straight line in any medium (like air or water).
• For example, when you see a car’s headlights or light coming through a window, you can see that the light travels in a straight line.
• This property of light moving in a straight line is called rectilinear propagation of light.
• Light travels very fast—it takes only 8 minutes to reach the earth from the sun and travels at a speed of 3 x 10⁸ meters per second.
• An opaque object completely blocks the path of light.

Activity 5.3

Aim: To show that light travels in a straight line.

Materials Required: A long straw (you can make it by joining two small straws).

Procedure:

• Hold the straw straight in front of a light source so one end faces the light.
• Look through the other end of the straw to see the light.
• Now, bend the straw and try to look through it again.

Observation:

• You can see the light when the straw is straight.
• You cannot see the light when the straw is bent.

Conclusion: This shows that light travels in a straight line.

Activity 5.4

Aim: To prove that light travels in a straight line.

Materials Required: Three sheets of cardboard of the same size, plasticine or clay, and a candle.

Procedure:

• Make a hole in the center of each cardboard piece.
• Place the cardboards on a table using clay or plasticine so they stand straight, and the holes are in a straight line.
• Light a candle and place it in front of the cardboards so the flame is in line with the holes.
• Look through the hole of the last cardboard and observe.
• Now, move one of the cardboards slightly and look again.

Observation:

• You can see the candle flame clearly through the holes when all three holes are in a straight line.
• You cannot see the flame if any of the cardboards are moved out of line.

Conclusion: Light always travels in a straight line.

The Consequences of Rectilinear Propagation of Light

Because light travels in a straight line, it has some effects:

• Formation of Shadows: When an opaque object is placed in the path of light, it blocks the light and makes a shadow.

• Occurrence of Eclipses: Eclipses happen when sunlight is blocked by a heavenly body, casting a shadow on another body

Formation of Shadows

• When you walk in the sun, you see a dark spot on the ground next to you. This dark spot is called a shadow.
• A shadow is made when something blocks the path of light coming from the sun or any light source.
• The shadow moves with you because you are blocking the light as you move.
• Shadows form because light travels in a straight line, and when something stops it, a dark patch is created.
• Not all objects make the same kind of shadow. Some objects block all the light, some block a little, and some let all the light pass through.
• An object that blocks all the light, like a book or a box, is called an opaque object. It makes a dark shadow.
• An object that blocks only some light, like butter paper, is called a translucent object. It makes a faint, light shadow.
• An object that lets all the light pass through, like glass, is called a transparent object. It does not make any shadow.
• To make a shadow, you need three things:
  • A source of light, like the sun or a torch.
  • An opaque object to block the light, like a ball or a book.
  • A screen, like a wall or the ground, where the shadow can be seen.

Activity 5.5

Aim: To show that you cannot see a shadow without a source of light.

Steps:

• Go to a very dark room where no light can come in.
• Make sure the room is completely dark.

What You See: You will not see any shadow because there is no light in the room.

Activity 5.6

Aim: To show that an opaque object is needed to make a shadow.

Steps:

• Go to an empty room with no objects in it.
• Turn on the lights in the room.
• Look around to see if there are any shadows.

What You See: You will not see any shadows because there is no opaque object to block the light coming from the source.

Activity 5.7

Aim: To show that a shadow can only be seen on a screen.

Things Needed: A torch and a chart paper.

Steps:

• Go to an open ground or a terrace in the evening when it is dark outside.
• Take 2-3 friends and an elder with you.
• Carry a torch and a chart paper with you.
• Ask one friend to stand in front of you at a distance.
• Turn on the torch so that its light falls on your friend's face.
• Make sure there is no tree or building behind your friend to block the light.

What You See: You will not see any shadow because there is no screen behind your friend to show the shadow.

Next Step: Now ask your second friend to hold the chart paper behind your first friend.

What You See Now: You will see the shadow of your first friend on the chart paper because the chart paper acts as a screen.

Conclusion: A shadow can only be seen on a screen, like the chart paper in this activity.

Characteristics of a Shadow

A shadow has some special features:

• A shadow is always formed on the opposite side of the light source. If the light comes from the left, the shadow will be on the right.
• The shadow has the same shape as the object that blocks the light. For example, if you hold a ball, the shadow will also look round like the ball.
• A shadow is always dark in color, no matter what color the object is or what color the light is.
• The clearness and length of a shadow depend on where the light source is:
  • In the morning and evening, when the sun is low, the shadow is long and not very clear.
  • At noon, when the sun is high, the shadow is short and darker.
• The size of a shadow depends on these things:
  • How far the light source is from the object.
  • How far the object is from the screen where the shadow is seen.
  • The size of the light source and the size of the object.
• When an opaque object is placed in the path of light, the shadow formed on the screen is very dark and has the same shape as the object. This dark region is called the umbra.
• If you use a small point source of light, like a tiny bulb, the shadow will have only the umbra (dark region).
• If you use a bigger light source, like a torch, the shadow will have two parts:
  • The darker region in the middle, which does not get any light, is called the umbra.
  • The lighter grey region around the umbra, which gets some light, is called the penumbra.
• A bigger light source makes a smaller umbra and a larger penumbra, which makes the shadow look fuzzy (not clear).
• The size of the umbra and penumbra also changes depending on:
  • The distance between the light source, the object, and the screen.
  • If the object is closer to the light source, the shadow will be larger and fuzzier.
  • If the object is closer to the screen, the penumbra will be smaller, and the shadow will be clearer.
  • If the screen is moved farther from the object, the umbra becomes smaller, and the penumbra becomes larger, making the shadow faint.

Activity 5.8

Aim: To show that a shadow is always dark in color.

Things Needed: Different colored objects like a pencil, rubber, ball, torch, and different colored plastic sheets (blue, red, green, etc.).

Steps:

• Take the different colored objects like a pencil, rubber, and ball.
• Place them between a wall and a torch, one by one.
• Turn on the torch and look at the shadow of each object on the wall.
• Now take the torch and cover its face with different colored plastic sheets, like blue, red, and green.
• Look at the shadows of the objects again with the colored light.

What You See: You will see that the shadow of each object is always dark, even when you use different colored objects or different colored lights.

Conclusion: A shadow is always dark in color. The color of the shadow does not change if you change the color of the light or the color of the object.

Activity 5.9

Aim: To see how the direction and length of a shadow change with the sun at different times of the day.

Things Needed: A long steel or wooden scale.

Steps:

• Take the long steel or wooden scale.
• Fix it in an open ground so it stands straight.
• Look at the shadow of the scale in the morning at 6 a.m., at noon at 12 p.m., and in the evening at 6 p.m.
• Notice the direction of the sun and the direction and length of the shadow at these times.
• Write down what you see.

What You See: You will see that the shadow is formed in the direction opposite to the sun. As the sun moves across the sky, the shadow moves too.

Conclusion: The length of the shadow changes depending on the position of the sun. At 6 a.m. and 6 p.m., the shadow will be longest because the sun is low. At 12 noon, the shadow will be shortest because the sun is directly above.

Did You Know?

Birds flying high in the sky do not cast a shadow on the ground because the shadow is very small. The umbra (dark part of the shadow) formed by the bird is small because the sun is a large light source. The penumbra (lighter part of the shadow) is large and very faint, so it cannot be seen on the ground.

Occurrence of Eclipses

• The earth, moon, and other heavenly bodies in our solar system do not make their own light.
• We can see them only when light from the sun falls on them and gets reflected back to us.
• The earth moves around the sun, and the moon moves around the earth in a straight line.
• Sometimes, the earth or moon blocks the sunlight, which creates a shadow on the other body.
• This blocking of light is called an eclipse.
• An eclipse happens because light travels in a straight line.
• There are two types of eclipses:
  • Solar eclipse: When the moon casts its shadow on the earth.
  • Lunar eclipse: When the earth casts its shadow on the moon.

Let's study these in detail:

Solar Eclipse

A solar eclipse happens when the moon comes between the sun and the earth during its movement. The moon fully or partially covers the sun, blocking the sunlight from reaching some parts of the earth. The shadow of the moon falls on the earth, making it dark during the daytime. A solar eclipse can only happen on a new moon day, when the moon is not visible from the earth.

There are three types of solar eclipses:

• Total solar eclipse: This happens when the umbra (darkest part) of the moon’s shadow falls on the earth. In this region, no light from the sun reaches, and the sun is completely hidden by the moon.
• Partial solar eclipse: This happens when the penumbra (lighter part) of the moon’s shadow falls on the earth. In this region, the moon is not in a straight line with the sun and the earth. Only a part of the sun is covered by the moon.
• Annular solar eclipse: This happens when the moon is exactly in line with the sun and earth, but the moon looks smaller than the sun. The moon does not completely cover the sun, so a bright ring of light called the corona is seen around the dark center of the moon.

Safety Tips:

Do not look at a solar eclipse with your naked eyes because it can harm your eyes. Use sunglasses, smoked screen, or a pinhole camera to safely see a solar eclipse.

Did You Know?

The longest annular solar eclipse of the millennium happened on 15 January 2010. It lasted for 11 minutes and 8 seconds. It was visible in parts of Africa, eastern Europe, the Middle East, and Asia. The next longest annular solar eclipse will happen on 23 December 3043.

Lunar Eclipse

We know that the moon does not make its own light. It shines because it reflects the sunlight. A lunar eclipse happens when the earth comes between the sun and the moon. The earth’s shadow falls on the moon, making it invisible for some time. A lunar eclipse can only happen on a full moon night, when the moon is fully visible from the earth. Every year, there are at least two lunar eclipses.

There are two types of lunar eclipses:

• Total lunar eclipse: This happens when the moon is completely inside the earth’s umbra (darkest shadow). The moon is not visible because no sunlight reaches it.
• Partial lunar eclipse: This happens when the moon passes through the earth’s penumbra (lighter shadow). Only a part of the moon is covered by the earth’s shadow, so it is partially visible.
• Penumbral lunar eclipse: When the moon passes through the earth's penumbra, it experiences penumbral lunar eclipse.

Did You Know?

We do not have an eclipse every month, even though the moon, earth, and sun come in a straight line often. This is because the orbit of the moon around the earth and the orbit of the earth around the sun are tilted at an angle of 5 degrees. Because of this tilt, the moon passes through the earth’s umbra or the earth’s shadow only during some full moon and new moon days. In a year, there can be a maximum of seven eclipses, which include a mix of solar and lunar eclipses.

Pinhole Camera

A pinhole camera is a simple device that makes images using light. It works because light travels in straight lines and passes through a tiny hole to form an image.

Image Formed in a Pinhole Camera

• Light rays from an object (XY) pass through the pinhole (O) to form an image (X’Y’) on the screen.
• Rays from point X travel along XO to X’, and from Y along YO to Y’, creating an inverted image.
• Rays between X and Y intersect at the pinhole, causing the image to flip upside down.

Characteristics of the Image:

• The image is inverted.
• The image is real and can be captured on a screen.
• Image size depends on:
  (a) The distance between the pinhole and the screen.
  (b) The distance between the object and the pinhole.
• Image Size Adjustment:
  • Increasing the distance between the pinhole and the screen enlarges the image.
  • Increasing the distance between the object and the pinhole reduces the image size.
• Magnification Formula:
  

• Additional Use: A pinhole camera can be used to safely observe a solar eclipse.

Advantages of a Pinhole Camera

• It does not need a lens to focus the light, so it is simple to make.
• Since there is no lens, the image does not have color distortion (colors stay true).

Disadvantages of a Pinhole Camera

• The image is not very clear or sharp, and it is hard to see small details.
• A pinhole camera can only take pictures of still objects (things that do not move), not moving objects.

Activity 5.10

Aim: To make a pinhole camera.

Materials Required:

• Two cardboard boxes of the same size.
• Black paint.
• Needle or awl pin.
• Aluminum foil.
• Grease-proof paper or butter paper.
• Black tape.
• Scissors.

Procedure:

• Paint the inside of both boxes with black paint and let them dry completely.
• Cut a small square in the center of one side of one of the boxes.
• Cut a square from the aluminum foil that is bigger than the square you made on the box.
• Use the needle or awl pin to make a tiny hole in the center of the aluminum foil.
• Tape the aluminum foil on the side of the box so the tiny hole is in the center of the square you cut.
• Cut the opposite end of this box and stick grease-proof or butter paper on it tightly to make a screen.
• Take the second box and cut a square on one side, big enough to look through, and cut off the opposite end.
• Attach the open end of the second box to the end of the first box that has the grease-proof paper.
• Seal any holes or gaps in both boxes using black tape to stop extra light from coming in.
• Your pinhole camera is ready to use.
• Take the pinhole camera outside and look through the viewing hole of the second box.
• Point the pinhole at a tree, plant, or any object, and see if you can see an upside-down image on the screen.

Conclusion: When I pointed the pinhole camera at a tree outside, I observed an upside-down image of the tree on the grease-proof paper screen. The image was clear but dim, and sealing all gaps with black tape ensured no extra light interfered.

Points To Remember

• Light does not need any medium (like air or water) to travel.
• The sun is a natural source of light energy.
• Things like candles, tube lights, and torches are artificial sources of light made by humans.
• The moon does not make its own light; it reflects the sun’s light and looks bright in the night sky.
• We can see objects that are not in direct sunlight because of the scattering of light.
• Light travels in a straight line in the same medium because it takes the shortest path from one point to another.
• The speed of light is very fast, about 3 x 10⁸ meters per second (m/s).
• Light from the sun takes about 8 minutes to reach the earth.
• Shadows and eclipses happen because light travels in straight lines (rectilinear propagation).
• Shadows form because an opaque object blocks light, and this happens due to the straight-line movement of light.
• An opaque object does not let light pass through it, so it makes a dark, sharp shadow of the same shape.
• A translucent object lets some light pass through, so it makes a faint shadow.
• A transparent object lets all light pass through, so it does not make any shadow.
• To make a shadow, we need a source of light, an opaque object, and a screen.
• The size and darkness of a shadow depend on the position of the light source, the object, and the screen.
• The size of the shadow also depends on the distance between the source and the object.
• The dark part (umbra) and the lighter part (penumbra) of a shadow change by changing the distance between the source and the screen.
• An eclipse happens because of the straight-line movement of light (rectilinear propagation).
• A solar eclipse happens when the moon comes between the sun and the earth during its revolution, blocking the sun’s light.
• In a solar eclipse, the moon’s shadow falls on the earth, making it dark in the daytime.
• A total solar eclipse happens when the moon fully covers the sun, and a bright ring called the corona is visible around the moon.
• A solar eclipse happens only on a new moon day (when the moon is not visible from earth).
• A lunar eclipse happens when the earth comes between the sun and the moon, and the earth’s shadow falls on the moon.
• A lunar eclipse happens on a full moon night (when the moon is fully visible).
• Every year, there are at least two lunar eclipses.
• In a pinhole camera, the tiny hole acts like a lens to form an image.
• The image made by a pinhole camera is upside down (inverted) and real (can be seen on a screen).
• The size of the image in a pinhole camera depends on the distance between the pinhole and the screen, and the object and the pinhole.
• A pinhole camera can be used to safely watch a solar eclipse without hurting your eyes.

Glossary

• Light: A form of energy that enables visibility by illuminating objects.
• Source of light: An object that emits light independently.
• Natural sources of light: Light sources created by nature, such as the sun.
• Artificial sources of light: Light sources made by humans, like bulbs or torches.
• Luminous objects: Objects that emit their own light, such as the sun or a candle flame.
• Non-luminous objects: Objects that do not emit light, like a table or book.
• Bioluminescence: Light produced by living organisms through chemical reactions, as seen in fireflies.
• Scattering of light: The spreading of light in various directions after striking a medium, like dust particles.
• Transparent object: An object that allows all light to pass through, such as glass.
• Translucent object: An object that permits some light to pass through, like butter paper.
• Opaque object: An object that blocks all light from passing through, such as a wooden block.
• Ray of light: A straight-line path along which light travels in a single direction.
• Beam of light: A collection of light rays traveling together in one direction.
• Parallel beam: A set of light rays moving parallel to each other without converging.
• Convergent beam: A set of light rays from different directions that meet at a single point.
• Divergent beam: A set of light rays originating from a single point and spreading outward.
• Rectilinear propagation of light: The property of light to travel in a straight line.
• Shadow: A dark region formed when an opaque object obstructs light.
• Umbra: The darkest part of a shadow where no light reaches.
• Penumbra: The lighter part of a shadow where some light reaches.
• Eclipse: An event where one object blocks the light of another, such as the moon blocking the sun.
• Solar eclipse: When the moon blocks the sun’s light, casting a shadow on Earth.
• Total solar eclipse: When the moon fully covers the sun, blocking all sunlight from reaching parts of Earth.
• Partial solar eclipse: When the moon covers only part of the sun, allowing some sunlight to reach Earth.
• Annular solar eclipse: When the moon, aligned with the sun, appears smaller and leaves a bright ring around the sun.
• Lunar eclipse: When Earth blocks sunlight, and the moon passes through Earth’s shadow.
• Total lunar eclipse: When the moon is entirely within Earth’s shadow, receiving no sunlight.
• Partial lunar eclipse: When the moon passes through part of Earth’s shadow, receiving some sunlight.
• Penumbral lunar eclipse: When the moon passes through the lighter part of Earth’s shadow, still receiving some sunlight.
• Pinhole camera: A simple device that uses a tiny hole to form images using light.
• Magnification: The ratio of the size of the image to the size of the object in a pinhole camera.