Light: Shadows And Reflections

Hello students, welcome to today's science lesson. I am so happy to see you all here, ready to learn about something truly wonderful – light! Today we are going to study Chapter 11 from your science textbook, and it is called "Light: Shadows and Reflections". This is a fascinating chapter that will help you understand some really interesting things about the world around you. Are you ready? Let us begin!

Imagine you are sitting in a village in Maharashtra, in the Western Ghat region, like our friend Keshav who we will read about in this chapter. At night, he sees hundreds of fireflies flashing their lights – is not that beautiful? Have you ever seen fireflies? They are such magical creatures, aren't they? But have you ever wondered how they produce light? Or think about the moonlight – have you ever looked at the moon and wondered whether it produces its own light or not? Well, students, we are going to answer all these questions and many more in this chapter.

Let us start by understanding what light is and where it comes from. This is a very important foundation for everything else we will learn.

## 11.1 Sources of Light

Students, look at the sun in the sky during the day. It gives us light and warmth, doesn't it? The Sun is the most important natural source of light for our Earth. The Sun gives out or emits its own light, and we call such objects luminous objects. The Sun is a luminous object. But wait, there are other objects too that emit their own light. Can you think of any? Yes, stars are also luminous – they produce their own light. Lightning is another example – when there is a thunderstorm, you see flashes of light, and that light is produced by lightning itself. Natural fire also produces light, and so do certain animals like fireflies and some deep-sea creatures.

Now, students, let us think about what humans used before electricity was invented. In ancient times, our ancestors learned to create fire. Can you imagine how difficult it must have been? They discovered how to make fire, and this fire was the earliest form of artificial lighting. Over time, humans learned to create fire using different fuels – animal fat, oil, wax, and gas. These were all sources of light that humans created artificially. Then came the invention of electricity, and with it, many different kinds of electric lights – bulbs, tubes, LEDs, and so on. Most of our lighting needs today are met by electric lighting.

Now, students, let us define some important terms. Objects that emit their own light are called luminous objects. The sun, stars, lightning, fire, and fireflies are all luminous objects. On the other hand, objects that do not emit their own light are called non-luminous objects. The moon is a non-luminous object. It does not produce its own light. It just reflects the light emitted by the Sun that falls on it. This is a very important point, students – the moonlight you see at night is actually sunlight that has bounced off the moon's surface and reached our eyes. So when Keshav wondered whether the moon produces its own light, the answer is no! The moon shines because it reflects sunlight. This is exactly what you learned in your Grade 6 chapter 'Beyond Earth' – all objects in our solar system shine only by reflecting the light of the Sun.

Let me give you a quick recap of what we have learned so far. Luminous objects are those that emit their own light, like the Sun, stars, fire, and fireflies. Non-luminous objects are those that do not emit their own light, like the Moon, the Earth, and all the objects around us that we see only because light from some luminous object falls on them and then reflects to our eyes.

Now, students, before we move on, let me tell you about a modern light source that is very important in India today. Light Emitting Diode lamps, called LED lamps, are modern light sources that consume much less power, are brighter, and last longer than traditional lamps. This not only reduces electricity bills but is also better for the environment. The Indian government has made substantial efforts to promote the use of LED lamps nationwide because of these advantages. However, at their end of life, LED lamps must be appropriately disposed or recycled, and not thrown in the garbage, because they contain some materials that can be harmful to the environment if not handled properly.

## 11.2 Does Light Travel in a Straight Line?

Now, students, here is a very interesting question. Does light travel in a straight line? How can we find out? Let us do some activities to investigate this.

For this activity, you will need three matchboxes, a torch, and a cardboard screen. First, take three matchboxes and make a hole in the inner tray of each matchbox, exactly at the same position. Then, arrange these three matchboxes in a straight line. Make sure that all three holes are exactly at the same height and are in a straight line. Now, place a torch on one side of the matchboxes, ensuring that its lamp is at the height of the holes. Place a cardboard screen on the other side of the matchboxes and obtain a bright spot on it. You may need to slightly adjust the heights of the boxes to get a clear spot.

Now, here is the interesting part. Move one of the matchboxes slightly to a side or up and down. Are you able to obtain the light spot on the screen now? When all the three holes are not in the same line, we could not obtain the light spot on the screen. This suggests that light travels in a straight line, students!

Let us try another way to confirm this. Take a long hollow pipe of some flexible material – you can use a flexible pipe that you might find at home or in a hardware store. Align it so that you can see a candle flame through it. Now, bend the pipe and try to see the candle flame again. Can you still see it? You could see the candle flame through a straight pipe but not through a bent pipe. This also shows that light travels in a straight line. Remember, students, always use a lighted candle under adult supervision.

There is another interesting activity we can do. Pass a laser beam through a beaker filled with water in which a drop of milk is added to make the laser beam easily visible. What do you observe? You will see that the beam of laser light inside water follows a straight path. This confirms that light travels in straight lines even in water. But remember, students, use a laser only under teachers' supervision. Never point the laser beam directly at anyone's eyes, as it can cause serious eye damage. Also, avoid using high-power lasers for this activity – a low-power laser pointer is sufficient.

Here is something fascinating, students – however, light can sometimes even bend around corners! This is something you will learn in the higher grades. For now, remember that under normal circumstances, light travels in a straight line.

So, let me recap what we learned in this section. Light travels in a straight line. We proved this through two activities – one with three matchboxes and holes in a straight line, and another with a straight pipe and a bent pipe. In both cases, light could pass through only when there was a straight path.

## 11.3 Light through Transparent, Translucent, and Opaque Materials

Now, students, let us think about what happens when an object comes in the path of light. Let us do an activity to find out.

Collect objects made of different materials. You will also need a torch. Let us make a table to record our observations. We need to classify materials into transparent, translucent, and opaque. You might remember learning about this in your Grade 6 chapter 'Materials Around Us'.

Let me explain what these terms mean. Transparent materials are those through which you can see clearly – like clear glass. Translucent materials are those through which you can see but not clearly – like frosted glass or tracing paper. Opaque materials are those through which you cannot see at all – like cardboard or a thick book.

Now, go to a dark room, turn on the torch, and place it at such a position that you get a spot of light from the torch on a wall. Or you may place a cardboard screen and get the spot of light on it. Now, predict what will happen if you hold an object in front of the light coming out of the torch. Would you continue to see the spot of light on the screen? Note your prediction. Then, actually place the object between the torch and the screen. Does light pass through the object? Note your observation. Repeat this for all the objects.

What conclusions can we draw, students? Light passes almost completely through transparent materials. Light passes partially through translucent materials. Light does not pass through opaque materials.

Now, what happens when an opaque object blocks the path of light? We see a dark patch on the screen or wall. This dark patch is called a shadow. We will learn more about shadows in the next section.

Let me recap this section. We learned that materials can be classified as transparent, translucent, or opaque depending on how much light passes through them. Transparent materials allow light to pass almost completely, translucent materials allow light to pass partially, and opaque materials do not allow light to pass at all.

## 11.4 Shadow Formation

Now, students, let us understand shadows in more detail. What did you see on the screen when an opaque object was placed in the path of light in the previous activity? You saw a dark patch, didn't you? Why was this dark patch formed? We know that light travels in a straight line. So, when an opaque object is placed in its path, light is blocked. The dark patch, where light does not reach, is the shadow.

We have noticed shadows of ourselves and other objects around us when we are in the Sun or under a light. Have you ever noticed your shadow on a sunny day? It follows you around! Also, you might have had fun making different shapes with shadows sometime – like making a dog shape with your hands and showing it on the wall in flashlight. That is shadow play!

Now, do translucent and transparent objects create shadows or not? Opaque objects form darker shadows. Translucent objects make lighter shadows. Even some transparent objects can create faint shadows! This is important, students – not only opaque objects but also some translucent and even transparent objects can create shadows, though they may be faint.

Let us do another activity to learn more about shadows. Collect some opaque objects of different shapes and sizes. Repeat the activity we did before, but this time, observe the shape and size of the shadow on the screen for each of the actions mentioned in the table.

Let me explain what to do. First, remove the screen – what do you observe? You cannot see a shadow because there is no screen to catch it. Then, remove the object – what do you see? You see only the light spot on the screen, no shadow. Then, switch off the torch – what do you see? There is no light at all, so no shadow. Now, keep the torch and screen fixed, and move the object closer to the screen – what happens to the shadow? It becomes smaller. Now, move the object closer to the torch – what happens? The shadow becomes larger. Finally, tilt the object – what happens to the shadow? The shadow also tilts and may change its shape.

What conclusions do we draw from this activity? We need a source of light, an opaque object, and a screen to observe a shadow. The walls, floor, ground, or any other surface acts as a screen for observing shadows in our daily life. The shape, size, and sharpness of the shadow depend on the position of the object relative to the light source and the screen. The shadows may give information about the object or we may not be able to guess the object at all. Changing the colour of opaque objects does not change the colour of the shadows – the shadow is always dark, regardless of the colour of the object.

Now, students, let me tell you something interesting. Shadow play, or shadow puppetry, has been a part of our cultural heritage for centuries. In this art form, flat cut-out figures called shadow puppets are placed between a light source and a screen. By moving the puppets and the light, puppeteers can create life-like movements, bringing the characters to life. Different regions in India have their own unique styles, like the Charma Bahuli Natya in Maharashtra, Keelu Bomme and Tholu Bommalata of Andhra Pradesh, Togalu Gombeyaata in Karnataka, Ravana Chhaya in Odisha, Tholpavakoothu in Kerala, and Bommalattam in Tamil Nadu. These are used not only for entertainment but also to communicate important messages to the community. Isn't that wonderful, students? Our ancestors used the properties of light and shadows to create such beautiful art forms!

Let me recap this section. Shadows are formed when an opaque object blocks light from falling on a screen. We need a source of light, an opaque object, and a screen to observe a shadow. The shape, size, and sharpness of the shadow depend on the relative positions of the light source, object, and screen. The colour of the shadow does not change with the colour of the object.

## 11.5 Reflection of Light

Now, students, here is something interesting. When we used a shiny object like a polished steel plate as the object in our activity, we got a shadow on the screen, but we also saw that there was a bright spot of light on the wall on the opposite side. Why was this so? This happens because the shiny surface reflects light!

Let us do an activity to understand reflection better. Find a shiny flat steel plate or a plane mirror – a mirror that is flat and not curved. Take it outside and let the sunlight fall on the shiny surface. What can you do to redirect light on the wall on which the sunlight is not falling directly? Turn the shiny plate or mirror in different directions to redirect the light onto a wall or a nearby surface. Do you see a spot of light on the wall? Does it mean that the shiny plate or mirror has changed the direction of light? Yes, it does! Now, tilt the shiny plate or mirror in different ways and observe the light spot on the wall. Does it change position? Notice how light always travels in straight lines and changes direction when it falls on the shiny plate or mirror.

What conclusion do we draw from this observation? A shiny surface or a mirror changes the direction of light that falls upon it. This change in direction of light by a mirror is called the reflection of light.

Let us do another activity to understand reflection better. Take a plane mirror with a stand, a torch, a comb, a sheet of white paper, and a strip of black paper. Using the black paper, close all openings of the comb except for one to make a thin slit. Spread a white paper on a table, hold the comb perpendicular to the sheet of paper and shine the torch light on the slit. Adjust the comb and torch slightly till you see a thin beam of light along the paper which has passed through the slit. Now, place the mirror in the path of the light beam while keeping the comb steady. What do you observe? The path of the light beam is changed after falling on the mirror. This is the reflection of light occurring at the mirror.

Now, students, when you look into a mirror, you can see your face. Is that also due to the reflection of light? Yes, it is! The light from your face falls on the mirror and reflects back to your eyes, which is why you can see yourself in the mirror.

Let me recap this section. A shiny surface or a mirror changes the direction of light that falls upon it. This change in direction is called reflection of light.

## 11.6 Images Formed in a Plane Mirror

Now, students, let us learn more about the images formed in a plane mirror. Look into the mirror. Do you see your face in it? What you see is a reflection of your face in the mirror. We also see reflections of other objects that are in front of the mirror. Let us try to find out more about this.

Take a plane mirror and a pen or some other object. Place the pen in front of the mirror as shown in the figure in your textbook. What do you see in the mirror? It appears as if a similar pen is placed behind the mirror. The pen which appears behind the mirror is the image of the pen formed by the mirror. The pen itself is the object.

Now, move the pen to different positions in front of the mirror and compare the sizes of the images of the pen at each position. Are the two sizes the same? The image formed by a plane mirror is of the same size as the object.

Again, move the pen to different positions in front of the mirror and observe if the image is upright at each position. Does the tip of the pen appear on top at each position? An upright image is called erect. An image formed by a plane mirror is erect.

Now, place a screen vertically behind the mirror. Move it around. Do you get the image on the screen? Repeat this by placing the screen in front of the mirror. The image formed by a plane mirror cannot be obtained on a screen. This is very important, students – the image in a plane mirror is virtual, meaning it cannot be captured on a screen.

Let us do another activity. Stand in front of a plane mirror and look at your image. Notice how far it appears to be from the mirror. Now, stand close to the mirror. Is the image also closer to the mirror? Stand at different distances from the mirror and notice how far the image appears to be from the mirror in each case. Do you find any relation between your distance from the mirror and the distance of your image from the mirror? You might have noticed that when you stand close to the plane mirror, the image also appears to be close to the mirror. The image appears to be far from the mirror when you stand far from the plane mirror. In fact, the image is at the same distance behind the mirror as the object is in front of it.

Now, here is something fun. Raise your left arm. Which arm does your image raise? Your image raises its right arm! Touch your right ear. Which ear does your image touch? Your image touches its left ear! You find that your left appears right in your image and your right appears left in your image. This type of perceived left-right reversal is called lateral inversion. There is lateral inversion in the images formed by a plane mirror.

Now, students, do you understand why 'AMBULANCE' is written in a special way on an ambulance vehicle? It is written as 'ƎƆИA˩UƎMA' so that when viewed from the rear-view mirrors of the vehicle ahead of the ambulance, it reads 'AMBULANCE' correctly! This is because of lateral inversion. When the letters are laterally inverted, they appear normal in the mirror, allowing drivers ahead to read it quickly and give way to the ambulance.

Let me tell you another interesting fact. When mirrors were invented is not known exactly. Earlier, mirrors were made by polishing stone or metal. When glass mirrors started being made, the art of making metal mirrors got lost gradually. However, it still survives, for example, in Kerala, where Aranmula Kannadi, a unique metal surface mirror has been made for centuries. It is a famous craft from Kerala.

Now, can we see an image of an object only in a mirror or are there some other ways as well? We will learn about another device – the pinhole camera – which can also form images.

Let me recap what we learned in this section. The image formed by a plane mirror is of the same size as the object, is erect, cannot be obtained on a screen, and is laterally inverted. The image appears to be at the same distance behind the mirror as the object is in front of it.

## 11.7 Pinhole Camera

Now, students, let us learn about a very interesting device called the pinhole camera. A pinhole camera is a device in which the light rays from an object pass through a tiny hole (a pinhole) and form an image on a screen.

Let us do an activity to understand this. Take a piece of cardboard and a candle. Make a small hole in the cardboard. In a dimly lit room, position the cardboard at a short distance from a screen. Place a lighted candle in front of it as shown in the figure in your textbook. What do you see on the screen? Light coming from the flame passes through the hole on the cardboard and forms an image of the candle flame on the screen. Do you notice anything surprising? The image of the candle flame is upside down, that is, inverted!

Why does this happen, students? It happens because light travels in a straight line. The light from the top of the candle flame goes straight through the pinhole and hits the bottom of the screen. Similarly, the light from the bottom of the candle flame goes straight through the pinhole and hits the top of the screen. That is why the image is inverted or upside down.

Now, let us make a pinhole camera that we can use outdoors. Take two boxes of cardboard such that one can slide into another with very little gap in between them. Cut open one side of each box. On the opposite face of the larger box, make a small hole in the middle. On the opposite face of the smaller box, cut out a square from the middle with a side of about 5 to 6 centimeters. Cover this opening with a thin translucent paper (like a tracing paper) to form a screen. Slide the smaller box inside the larger one in such a way that the side with the tracing paper is inside.

Hold the pinhole camera with the pinhole facing the object and look through the open side of the smaller box. Cover your head and the camera with a dark cloth. Look at a distant object, like a tree or building, in bright sunlight and move the smaller box forward or backward until an image appears on the tracing paper.

Do the images seen in the camera show the colours of the objects on the other side? Yes, they do! Are the images erect or upside down? They are upside down, or inverted.

So, students, a pinhole camera gives an upside down image. On the other hand, there is lateral inversion in the image formed by a mirror but it is not upside down. We will learn more about this in higher grades.

Let me recap this section. A pinhole camera is a device that forms an inverted image of an object on a screen. Light passes through a tiny hole and travels in a straight line to form the image.

## 11.8 Making Some Useful Items

Now, students, after having learnt that light travels in a straight line and is reflected by mirrors, it is time to create some useful items based on this learning. Let us learn about two interesting devices – the periscope and the kaleidoscope.

### 11.8.1 Periscope

We can make a simple periscope by placing two plane mirrors in a Z-shaped box as shown in the figure in your textbook. Reflection from two mirrors enables us to see objects that are not visible directly. Periscopes are used in submarines, tanks, or by soldiers to see outside their bunkers. You may use it to look ahead when standing behind taller friends. Imagine you are in a crowd and cannot see over people's heads – if you had a periscope, you could see over them! Isn't that useful?

### 11.8.2 Kaleidoscope

Now, let us make a kaleidoscope. Get three rectangular plane mirror strips of equal width and join them together in a triangular manner. You may use three strips of thick reflective paper instead of mirrors. Fix these in a circular tube of thick chart paper. On one end of the tube, fix a transparent plastic sheet using a rubber band or an adhesive tape. Place several broken pieces of coloured bangles or beads on this, and cover it with a tracing paper using a rubber band or an adhesive tape.

When you peep through the open side, you view a beautiful pattern. Even if you leave both sides of the kaleidoscope open and point it towards a tree or other objects, you see beautiful patterns. An interesting feature of the kaleidoscope is that one always gets to see a different pattern every time the kaleidoscope is turned about. Since there are three mirrors, and multiple images (due to reflections of reflections), many interesting patterns are formed. Designers and artists often use kaleidoscopes to get ideas for new patterns. Would not it be fun to make one and see the beautiful patterns?

Now, students, we have completed learning all the concepts in this chapter. Let me give you a summary of everything we have learned.

## In a Nutshell

Let me summarize all the important points from this chapter:

Objects that emit their own light are called luminous objects. The Sun, stars, fire, and fireflies are examples of luminous objects.

Light travels in a straight line. We proved this through activities with matchboxes and pipes.

Light passes almost completely through transparent materials. Light passes partially through translucent materials. Light does not pass through opaque materials.

A shadow is formed when light is blocked by an object. Opaque objects form darker shadows. Translucent objects make lighter shadows. Some transparent objects can create faint shadows.

The change in the direction of light by a mirror is called reflection of light.

The image formed by a plane mirror is of the same size as the object, is erect, cannot be obtained on a screen, and is laterally inverted.

A pinhole camera creates an inverted image of an object on a screen.

Now, students, let us solve the exercises from your textbook. I will go through each question one by one and explain the answer.

## Let Us Enhance Our Learning

Question 1: Which of the following are luminous objects? Mars, Moon, Pole Star, Sun, Venus, Mirror

Let us think about each one, students. Mars is a planet – it does not produce its own light, it reflects sunlight, so it is not luminous. The Moon – we learned that it does not produce its own light, it reflects sunlight, so it is not luminous. The Pole Star – this is a star, and stars do produce their own light, so it is luminous! The Sun – obviously luminous! Venus – this is a planet, it reflects sunlight, so not luminous. A mirror – it reflects light but does not produce its own light, so not luminous. So the luminous objects from this list are the Pole Star and the Sun.

Question 2: Match the items in Column A with those in Column B.

Column A has: Pinhole camera, Opaque object, Transparent object, Shadow Column B has: Blocks light completely, The dark region formed behind the object, Forms an inverted image, Light passes almost completely through it

Let us match them, students. Pinhole camera forms an inverted image – so that matches with "Forms an inverted image". Opaque object blocks light completely – so that matches with "Blocks light completely". Transparent object – light passes almost completely through it – so that matches with "Light passes almost completely through it". Shadow is the dark region formed behind the object – so that matches with "The dark region formed behind the object".

Question 3: Sahil, Rekha, Patrick, and Qasima are trying to observe the candle flame through the pipe as shown in the figure. Who can see the flame?

Looking at the figure, students, we can see that the pipe is bent. We learned earlier that light travels in a straight line and cannot pass through a bent pipe. So, none of them can see the flame because the pipe is bent and blocks the straight path of light. The answer is that none of them can see the flame.

Question 4: Look at the images shown in the figure and select the correct image showing the shadow formation of the boy.

This question refers to a figure in your textbook. The correct shadow would be formed on the opposite side of the light source. If the light is on the left side of the boy, the shadow would be cast to the right. The shadow would be dark and roughly the same shape as the boy, but it would be inverted if the light is above him. Without the actual figure, I cannot tell you exactly which option is correct, but remember the key points: the shadow is formed on the opposite side of the light source, and it is dark because light is blocked by the boy.

Question 5: The shadow of a ball is formed on a wall by placing the ball in front of a fixed torch as shown in the figure. In scenario (i) the ball is closer to the torch, while in scenario (ii) the ball is closer to the wall. Choose the most accurate representation of the shadows formed in both scenarios from the options provided.

This is about the size of the shadow, students. When the ball is closer to the torch, the shadow on the wall will be larger. When the ball is closer to the wall, the shadow will be smaller. This is because when the object is closer to the light source, it blocks more of the light, creating a larger shadow. So in scenario (i), the shadow should be larger, and in scenario (ii), the shadow should be smaller.

Question 6: Based on the figure, match the position of the torch in Column A with the characteristics of the ball's shadow in Column B.

Column A has: If the torch is close to the ball, If the torch is far away, If the ball is removed from the set-up, If two torches are present in the set-up on the left side of the ball

Column B has: The shadow would be smaller, The shadow would be larger, Two shadows would appear on the screen, A bright spot would appear on the screen

Let us match them, students. If the torch is close to the ball – the shadow would be larger. If the torch is far away – the shadow would be smaller. If the ball is removed from the set-up – there would be no shadow at all, but wait, the option says "Two shadows would appear on the screen" – this is not correct. If the ball is removed, there would be no shadow. But if there are two torches, then we would get two shadows. So "If two torches are present in the set-up on the left side of the ball" matches with "Two shadows would appear on the screen". And "A bright spot would appear on the screen" – this does not seem to match any of the options in Column A. Let me reconsider. Actually, if the ball is removed, we would just see the light spot on the screen, not a shadow. So the matching would be: If the torch is close to the ball – the shadow would be larger. If the torch is far away – the shadow would be smaller. If two torches are present – two shadows would appear.

Question 7: Suppose you view the tree shown in the figure through a pinhole camera. Sketch the outline of the image of the tree formed in the pinhole camera.

Remember, students, a pinhole camera forms an inverted image. So the image of the tree would be upside down. The top of the tree would appear at the bottom, and the bottom of the tree would appear at the top. You need to sketch this in your notebook.

Question 8: Write your name on a piece of paper and hold it in front of a plane mirror such that the paper is parallel to the mirror. Sketch the image. What difference do you notice? Explain the reason for the difference.

When you write your name on a paper and hold it in front of a plane mirror, the image you see in the mirror will be laterally inverted. This means that left and right appear to be swapped. Your name will appear to be written backwards. This happens because of lateral inversion in plane mirrors. The mirror reflects the light back, and our brain interprets the reflection as if it were a real object behind the mirror, causing the left-right reversal.

Question 9: Measure the length of your shadow at 9 AM, 12 PM, and 4 PM with the help of your friend. Write down your observations: (i) At which of the given times is your shadow the shortest? (ii) Why do you think this happens?

This is a practical activity, students. At 12 PM, which is noon, the sun is directly overhead. At this time, your shadow will be the shortest because the light from the sun is coming from directly above you, so very little light is blocked by your body. In the morning and evening, the sun is at a lower angle, so your shadow is longer because more of the light is blocked. So the shortest shadow will be at 12 PM.

Question 10: On the basis of following statements, choose the correct option.

Statement A: Image formed by a plane mirror is laterally inverted. Statement B: Images of alphabets T and O appear identical to themselves in a plane mirror.

Let us analyze this, students. Statement A is true – we learned that images formed by a plane mirror are laterally inverted. Statement B: Let's think about the alphabet T. If you look at T in a mirror, it appears as T – it looks the same! What about O? O also looks the same in a mirror – it is symmetric. So both T and O appear identical to themselves in a plane mirror. So Statement B is also true. Therefore, both statements are true. The correct option is (i) Both statements are true.

Question 11: Suppose you are given a tube of the shape shown in the figure and two plane mirrors smaller than the diameter of the tube. Can this tube be used to make a periscope? If yes, mark where you will fix the plane mirrors.

Yes, students, this tube can be used to make a periscope. The tube is Z-shaped, which is exactly the shape we need for a periscope. You would fix the two plane mirrors at the bends in the tube, as shown in the figure in your textbook. The mirrors should be placed at 45-degree angles so that the light is reflected properly from one end to the other.

Question 12: We do not see the shadow on the ground of a bird flying high in the sky. However, the shadow is seen on the ground when the bird swoops near the ground. Think and explain why it is so.

This is a great question, students. When a bird is flying high in the sky, the light from the sun is spread over a large area. The bird is relatively small compared to the distance between it and the ground. The shadow formed by the bird is very faint and spread out, so we cannot see it clearly. Also, the sun's light comes from a large area in the sky (it is not a point source), so the shadow is not sharp. When the bird swoops near the ground, it is much closer to the ground, so it blocks more of the light from the sun. The shadow becomes larger and darker, making it visible. Also, when the bird is closer to the ground, the shadow is more defined.

Now, students, let us look at the Exploratory Projects. These are optional activities you can do to learn more.

First project: Have you ever seen a firefly where you live? If no, ask your elders if fireflies were seen earlier in your region. If yes, find out the reasons for their not being seen anymore. Develop a story about it. This is a research project about fireflies in your area.

Second project: Repeat Activity 11.4, but this time cover the face of the torch with a coloured transparent paper and observe the colour of the shadow. Repeat this using transparent paper of different colours. Report your conclusions. This will help you understand whether the colour of light affects the colour of the shadow.

Third project: A plane mirror forms only a single image of an object. But what will happen if two or more mirrors are kept at an angle with each other or parallel to each other? Find out by placing two mirrors as shown in the figure. This will help you understand multiple reflections.

Fourth project: You are given a small piece of a plane mirror. Can this piece form an image of an object much larger than the mirror, like a large tree? Think and predict. Then carry out the activity. This will help you understand how mirrors can form images of objects larger than themselves.

Now, students, before we end, let me tell you one more fascinating fact. Light emitted by the Sun takes about 8 minutes and 20 seconds to reach the Earth. Hence, if the Sun were to suddenly stop emitting light, we would not know that for another 8 minutes and 20 seconds. Isn't that amazing? The sun is very far away from us, so even though light travels at a very fast speed (about 300,000 kilometers per second), it takes more than 8 minutes to reach us from the sun.

## Complete Summary

Now, students, let me give you a complete summary of everything we learned in this chapter.

We started by learning about sources of light. Objects that emit their own light are called luminous objects, like the Sun, stars, fire, and fireflies. Objects that do not emit their own light are called non-luminous objects, like the Moon, which reflects sunlight.

Then we learned that light travels in a straight line. We proved this through activities with matchboxes and pipes.

We learned about transparent, translucent, and opaque materials. Transparent materials allow light to pass almost completely, translucent materials allow light to pass partially, and opaque materials do not allow light to pass at all.

We learned about shadow formation. Shadows are formed when an opaque object blocks light from falling on a screen. We need a source of light, an opaque object, and a screen to observe a shadow. The shape, size, and sharpness of the shadow depend on the relative positions of the light source, object, and screen.

We learned about reflection of light. A shiny surface or a mirror changes the direction of light that falls upon it. This is called reflection.

We learned about images formed in a plane mirror. The image is of the same size as the object, is erect, cannot be obtained on a screen, and is laterally inverted. The image appears to be at the same distance behind the mirror as the object is in front of it.

We learned about the pinhole camera, which forms an inverted image of an object on a screen.

Finally, we learned about useful items like periscopes and kaleidoscopes that are based on the properties of light.

That, students, is everything in this chapter. I hope you enjoyed learning about light, shadows, and reflections as much as I enjoyed teaching you. Remember, science is all around us, and if you observe carefully, you will see these concepts in action everywhere – in the shadows you see on a sunny day, in the mirror you use every morning, and in the beautiful patterns of a kaleidoscope. Thank you for your attention, and keep exploring the wonderful world of science!