Light: Mirrors And Lenses

Hello students, welcome to today's science lesson. I'm so happy to see you all here, ready to learn something fascinating about the world around us. Today, we are going to study Chapter 10 from your Science book — Light: Mirrors and Lenses. This is such an interesting chapter because it explains how we see images in different types of mirrors and lenses, and you'll be amazed to know how these concepts are used in our daily lives. So let's begin our journey into the world of light and reflections.

So students, let me start with a story. During the summer holidays, a girl named Meena went to a science centre with her family. While her parents were exploring a section on saving water and electricity, Meena and her brother wandered off to look around. In one corner, Meena noticed a row of unusual, curved mirrors. Curious, she stepped closer and looked into one. Her face appeared unusually large, while her brother, standing a little farther away, looked upside down! At another mirror, she saw a tiny version of herself. Meena was very puzzled.

She remembered doing activities with a plane mirror earlier, where the image formed was of the same size as the object and was erect. But these mirrors were different! A guide from the science centre smiled and explained, "These are not plane mirrors. These are spherical mirrors. When the mirror is curved inward or outward, your image looks different in them!" Meena's curiosity grew, and she decided to learn more about these spherical mirrors from her teacher. And that, dear students, is exactly what we are going to learn today.

So let's start with the first question: What are spherical mirrors?

Now, students, you all must have used a metallic spoon at home. Let me ask you — have you ever looked at your reflection in the curved surface of a spoon? Let me guide you through a simple activity that we can imagine doing.

Take a shiny metallic spoon and hold its curved surface close to your face. Can you see your image in it? Yes, you can! Now, notice the image of your face. Is it different from the image you see in a plane mirror? Absolutely yes! While observing the image, slowly move the spoon away from your face. Do you observe any change in the image? Yes, the image changes as you move the spoon. Now, flip the spoon and repeat the same steps with the other side.

What did you notice? When you looked at the inner side of the spoon which is curved inwards, you must have observed that the image was inverted — that means upside down. When you looked at the outer side of the spoon which bulges outwards, the image of your face was erect but smaller in size. So students, the shiny metallic spoon acted like a mirror, and you could see your image in it!

Now, curved mirrors like the spoon can also be specially made. These are called spherical mirrors. Mirrors whose reflecting surfaces are spherical are called spherical mirrors. The word "spherical" means having the shape of a sphere. So a spherical mirror is shaped like a part of a hollow glass sphere.

Now, students, there are two types of spherical mirrors. The reflecting surface of the spherical mirror may be curved inwards or outwards. Let me explain both types clearly.

A spherical mirror which has a reflecting surface that curves inwards — like the inner side of a bowl — is called a concave mirror. Imagine a cave that goes inward; that's exactly what concave means. So a concave mirror curves inward. Its reflecting surface is like the inside of a spoon's bowl.

On the other hand, a spherical mirror which has a reflecting surface that curves outwards — like the outer surface of a ball — is called a convex mirror. Convex means bulging outward. So a convex mirror curves outward, like the back of a spoon.

In diagrams, we represent these mirrors like this: the non-reflecting surface of the mirror is shown as shaded. For a concave mirror, the reflecting surface curves inward, and for a convex mirror, the reflecting surface curves outward.

Now, students, let me tell you an interesting point. The shape of a spherical mirror is such that it can be thought of as a part of an imaginary hollow sphere. However, remember that spherical mirrors are not made by slicing a hollow glass sphere. Instead, they are created by grinding and polishing a flat glass piece into a curved surface. If a reflective coating like a thin layer of aluminium is applied on the outer curved surface, it forms a concave mirror. If the coating is applied on the inner curved surface, it forms a convex mirror. This is how these mirrors are actually made in factories.

Now, let's do an activity to distinguish between concave and convex mirrors. Place concave and convex mirrors on a table with their reflecting surfaces facing upwards. Now view them from the side, keeping your eye at their level, to identify whether the reflecting surface is curved inwards or outwards. How can we distinguish between concave and convex mirrors from their side view? Students, if you look from the side, a concave mirror will appear to curve inward, like a cave, while a convex mirror will appear to bulge outward. That's how you can tell them apart.

Now, let's move to our next important topic: What are the characteristics of images formed by spherical mirrors?

Let's do another activity. Take a concave mirror, a convex mirror, two small wooden blocks to place the mirrors in an upright position, and a small toy or some other object. Place the two mirrors side by side in an upright position on a table. Keep the object in front of them at a small distance, about 3 to 4 centimetres away. What kind of images do you see in each mirror? Are the images of the same size as the object? Are they erect? Do you see lateral inversion in the images? Write down your observations in your notebook.

Now, slowly move the object away from the mirrors. What changes do you see in the images in both the mirrors? Do the images become smaller or larger? Do they continue to be erect? Again, note down your observations. Repeat the steps with each mirror individually. Analyse your observations and draw conclusions.

Now, students, let me tell you what we observe in this activity. In the concave mirror, when the object is placed close to the mirror, the image is erect but larger than the object in size, that is, enlarged. However, when the object is moved farther away, the image becomes inverted. Initially, the image is enlarged in size and then keeps getting smaller. In case of a convex mirror, the image is always erect and smaller than the object, that is diminished. However, the size of the image decreases slightly as the object is moved away from the convex mirror.

So students, this activity shows that spherical mirrors behave differently from plane mirrors. A plane mirror always forms an erect image of the same size as the object. However, in concave and convex mirrors, the size of the image changes as the distance of the object from the mirror changes. In addition, in the case of a concave mirror, the image also gets inverted when the object is taken away from the mirror. And importantly, lateral inversion of the image is seen in all three types of mirrors — plane, concave, and convex. Lateral inversion means that if you raise your right hand, the image appears to raise its left hand. This happens in all mirrors.

Now, here's an interesting idea: We can identify whether a mirror is plane, concave, or convex by looking at the images of an object formed in them! If the image is the same size as the object and erect, it could be a plane mirror. If the image is enlarged and erect when close, but becomes inverted when far, it's a concave mirror. If the image is always erect but smaller than the object, it's a convex mirror.

Now, where do we find concave and convex mirrors being used in our surroundings? Let me give you some real-life examples.

The reflectors of torches and headlights of cars and scooters are concave in shape. Have you ever noticed a dental mirror used by a dentist for inspecting teeth? It is a concave mirror which provides an enlarged view of teeth when held close to the teeth inside the mouth. This helps the dentist see your teeth properly.

Now, look at the side-view mirrors on vehicles. These mirrors are convex. They always form an erect image of the traffic behind and smaller than the actual vehicles. Also, since the convex mirror is curved outside, it provides a much wider area of the road behind. This is why you see the warning "Objects in mirror are closer than they appear" on side-view mirrors — because the image is smaller than the actual object, making it appear farther away! Further, such convex mirrors are installed at road intersections or sharp bends to provide drivers from both sides the visibility of the other side and prevent collisions. Convex mirrors are also installed in big stores to monitor a large area to deter thefts.

So students, now you understand why the warning is written on side-view mirrors. The convex mirror makes objects appear smaller and farther away than they actually are, so the warning is necessary for safety.

Now, let's learn about the laws of reflection. We have observed images formed by three types of mirrors — plane, concave, and convex. But are there any laws which govern the image formation? Yes, there are! Let's find out.

Let us repeat an activity which you might have done earlier in grade 7, but this time we will extend it further. Do you remember doing the activity for observing the reflection of a beam of light from a plane mirror?

Here's the activity. Collect a plane mirror with a stand, a torch, a comb, a paper clip to hold the comb upright, a sheet of white paper, and a strip of black paper. As you did earlier, make a thin slit by covering all openings of the comb using black paper, except for one in the middle. Spread a sheet of white paper on a table. Place the plane mirror upright on it. Using the thin slit and torch, obtain a thin beam of light along the paper and adjust it to fall upon the mirror.

Now, move the slit and torch slightly so that the beam of light falls at a different angle on the mirror. Does the reflected beam of light also shift? Yes, it does! Make the beam of light fall on the mirror at different angles and observe how the direction of the reflected beam changes.

To understand this better, let us draw this on a paper, step by step. But before doing that, let us learn how to represent light. We often represent light by straight lines with arrows, or rays. Rays indicate the path along which light travels. Do you remember learning earlier that light travels along a straight line? Yes, that's correct.

Now, draw a line showing the position of the plane mirror. Also, draw lines with arrows indicating the beam of light falling on the mirror and the reflected beam of light.

The ray of light that falls on the mirror is called the incident ray. The ray of light that comes back from the mirror is called the reflected ray.

Now remove the mirror. From the point where the incident ray strikes the mirror, draw a line making an angle of 90 degrees to the line representing the mirror. This line is known as the normal to the reflecting surface at the point of incidence.

The angle between the normal and the incident ray is called the angle of incidence. The angle between the normal and the reflected ray is known as the angle of reflection.

On your drawing, measure the angle of incidence and the angle of reflection and note it in a table. Repeat the activity several times by changing the angle of incidence. Finally, let the incident beam fall on the mirror along the normal and observe the direction of the reflected beam. What would be the angle of incidence and angle of reflection in this case? Both the angles would be zero in this case.

Now, what do you notice? Both angles in the table are nearly equal! If done carefully, the experiment shows that the angle of incidence is equal to the angle of reflection. This is the first law of reflection.

Now, let's do another activity to understand the second law of reflection. Use the same setup as before, but place a stiff sheet of chart paper flat on a table such that part of it extends beyond the edge of the table. Shine a beam of light on the mirror placed on the sheet and observe the reflected beam on the extended portion. Now, bend the extended part of the sheet along the edge of the table. Do you still see the reflected beam on the extended portion? Flatten the paper again and observe.

The reflected beam disappears when the sheet is bent but reappears when it is flattened again. This shows that the reflected beam lies in the same plane as that of the incident beam. Bending the sheet creates a new plane, breaking this alignment.

So students, the second law of reflection is: The incident ray, the normal to the mirror at the point of incidence, and the reflected ray, all lie in the same plane.

So to summarize, the two laws of reflection are: First law: The angle of incidence is equal to the angle of reflection. Second law: The incident ray, the normal to the mirror at the point of incidence, and the reflected ray, all lie in the same plane.

Now, are these laws applicable to spherical mirrors also? Yes, students! The laws of reflection are valid for all kinds of mirrors — plane and spherical. But if multiple parallel rays fall on spherical mirrors, we observe something interesting.

Let's do another activity. Collect a plane mirror, a concave mirror, a convex mirror, stands for mirrors, a torch, a comb, and a paper clip to hold the comb upright. Use the same setup as before, but instead of a single slit, leave many openings of the comb uncovered to obtain multiple parallel beams of light. Let the multiple parallel beams of light fall upon the plane mirror, concave mirror, and convex mirror, one by one. Observe the reflected beams.

When multiple parallel beams of light fall upon a plane mirror, the multiple reflected beams are also parallel. However, when multiple beams of light fall upon a concave mirror, the multiple reflected beams get closer, that is, they converge. Whereas, in the case of a convex mirror, the multiple reflected beams spread, that is, they diverge.

So students, in the case of spherical mirrors, even though each ray of light follows the laws of reflection, the curved surface of spherical mirrors causes the parallel beam of rays to either converge or diverge on reflection depending on the shape of the mirror. The concave mirror converges a light beam while the convex mirror diverges it. This is indeed interesting!

Now, since the concave mirror converges the light beam, wouldn't light get concentrated in a small area? Yes, it would! Let's do an activity to see this effect.

Safety first, students! Always perform this activity under the supervision of a teacher or an adult. Do not look towards the Sun or into the mirror reflecting the Sun. Focus the reflected light only on a piece of paper, not towards anyone's face or eyes.

Take a concave mirror and a sheet of thin paper or newspaper. Hold the concave mirror with its reflecting surface facing the Sun. Direct the light of the Sun reflected by the mirror on the sheet of paper. Adjust the distance of the paper until you get a sharp bright spot on it. Hold the mirror and the sheet of paper steady for a few minutes. Does the paper start to burn producing smoke?

Yes, students! The bright spot is formed on the paper because light from the Sun, after reflection from the mirror, gets concentrated on this point. This produces sufficient heat at this point which can ignite the paper. This is exactly how we can start a fire using a concave mirror and sunlight!

Now, devices which concentrate sunlight into a small area, using mirrors and lenses, are called solar concentrators. The concentrated sunlight is used to heat a liquid to produce steam which can be used to generate electricity or for providing heat for various purposes, such as large scale cooking or for solar furnaces. Solar furnaces are even used for melting steel! This is amazing, isn't it?

Now, students, let's move on to a new topic: What is a lens?

Imagine looking through a flat transparent glass window pane — all objects look the same size and shape. But would those objects continue to look the same if the surface of the transparent material is curved? We explored the images of an object formed by curved mirrors. But how do objects look when viewed through transparent materials with curved surfaces?

Let's do an activity. Collect a flat strip of glass or clear plastic, such as a flat scale, few drops of oil, a dropper, water, and a paper or book with some text printed on it. Spread a few drops of oil on the surface of glass or plastic strip and rub it to leave a very thin coating. You can also use wax instead of oil. Using a dropper or your finger, place a small drop of water on the oiled or waxed spot. The oil or wax helps the water form a nice round drop.

Now, examine the water drop. What is the shape of its surface? Is it flat or curved inward or curved outward? The surface of the water drop is curved outside. Now, place the paper underneath the glass or plastic strip such that the text is directly under the water drop. Now, look down through the water drop at the text below. Do you find some change in the size of the letters just below the water drop? Do they look enlarged or smaller?

The letters under the water drop look different — they might appear larger than the letters nearby! The curved surface of the water drop made the size of the text look different. This curved drop of water is acting like a simple lens. Have you seen a magnifying glass? It is also a lens that helps in reading small print by making the letters appear bigger.

So students, a lens is a piece of transparent material, usually made of glass or plastic, which has curved surfaces. Like mirrors, lenses can also be convex or concave.

A lens which is thicker at the middle as compared to the edges is called a convex lens. Imagine a lens that is like a magnifying glass — thicker in the centre. That's a convex lens.

A lens which is thicker at the edges as compared to the middle is called a concave lens. This lens is thinner in the middle and thicker at the edges, like a bowl turned upside down.

Unlike mirrors, lenses allow light to pass through them, and we see things through a lens rather than in a lens. With mirrors, we see the reflection on the surface. With lenses, light passes through them and we see the object through the lens.

Now, what changes can be seen in the objects when viewed through lenses? Let's do an activity.

Collect a convex lens, a concave lens, a lens holder, and a small object. Take the convex lens and place it upright using its holder. Place the object behind the convex lens. Look at the object through the lens from the other side of the lens and note your observations in your notebook. Now slowly move the object farther from the lens and keep observing how the image changes. How does the distance of the object from the convex lens affect how it looks? Now repeat the steps using a concave lens. Analyse your observations recorded in your notebook and compare the images seen through both lenses. What conclusions do you draw?

Students, here's what we observe. When an object is placed behind a convex lens at a small distance from it and seen through the lens, the object appears erect and enlarged in size. As the distance between the object and the convex lens increases, the object appears inverted. It is initially enlarged in size and then diminishes in size. An object placed behind a concave lens and seen through the lens always appears erect and diminished in size. Its size changes as its distance from the lens increases, but it always remains erect and smaller than the actual object.

Now, do lenses also converge or diverge the light beam? Let's investigate.

Collect a thin transparent glass plate, a convex lens, a concave lens, a torch and a comb to obtain multiple parallel beams of light, a paper clip to hold the comb upright, two identical books, and sheets of white paper. Using two books placed adjacent to each other, fix the glass plate or lens upright in between them. Spread paper sheets on both books. Now let the multiple parallel beams of light fall upon the thin glass plate, convex lens, and concave lens one by one. Does the parallel beam of light pass through as it is in all three cases?

The light beam passes through the thin glass plate as it is. The convex lens converges the light falling on it while the concave lens diverges the light. A convex lens is also called a converging lens while a concave lens is called a diverging lens.

Now, since convex lens converges a light beam, can it also burn a paper? Yes, it can! Let's investigate.

Repeat the activity we did with the concave mirror by putting a convex lens in the path of sunrays in place of a concave mirror. Could you burn the paper? Yes, you could! The convex lens focuses the sunlight to a point, creating enough heat to burn the paper. This is similar to how we used the concave mirror.

Now, where are lenses used? Lenses are important and are used everywhere around us. The eyeglasses that people wear to help them see clearly are lenses. Cameras, telescopes, and microscopes all use lenses to work. Even our eye has a convex lens inside it. It is quite an amazing lens that can change its shape, which is what allows us to read a book or see something far away. This is called accommodation — the lens in our eye changes shape to focus on objects at different distances.

Now, students, let's summarize what we've learned so far. Here are the key points:

Image formed by a concave mirror can be enlarged, diminished, or of the same size as the object, and it may be erect or inverted, depending upon the distance of the object from the mirror.

Image formed by a convex mirror is always erect and diminished in size.

Two laws of reflection are: The angle of incidence is equal to the angle of reflection. And the incident ray, the normal to the mirror at the point of incidence, and the reflected ray, all lie in the same plane.

The laws of reflection are valid for all kinds of mirrors — plane, concave, and convex.

A concave mirror converges the light beams while a convex mirror diverges it.

Image formed by a convex lens can be enlarged, diminished, or of the same size as the object, and it may be erect or inverted, depending upon the distance of the object from the lens.

Image formed by a concave lens is always erect and diminished in size.

A convex lens converges the light beams while a concave lens diverges it.

Now, students, let's solve some questions to check our understanding. Let's go to the "Keep the curiosity alive" section.

Question 1: A light ray is incident on a mirror and gets reflected by it. The angle made by the incident ray with the normal to the mirror is 40 degrees. What is the angle made by the reflected ray with the mirror?

Now students, remember the first law of reflection: angle of incidence equals angle of reflection. The angle made by the incident ray with the normal is 40 degrees, so the angle of reflection is also 40 degrees. But the question asks for the angle made by the reflected ray with the mirror. Now, what is the relationship between the angle of reflection and the angle with the mirror? The angle with the mirror plus the angle of reflection equals 90 degrees, because the normal is perpendicular to the mirror. So if the angle of reflection is 40 degrees, then the angle with the mirror is 90 minus 40, which equals 50 degrees. So the answer is 50 degrees. The correct option is (ii) 50 degrees.

Question 2: Fig. 10.22 shows three different situations where a light ray falls on a mirror. In the first case, the light ray falls along the normal. In the second case, the mirror is tilted, but the light ray still falls along the normal to the tilted surface. In the third case, the mirror is tilted, and the light ray falls at an angle of 20 degrees from the normal. Draw the reflected ray in each case. What is the angle of reflection in each case?

Students, let's think about this carefully. In the first case, the light ray falls along the normal. This means the angle of incidence is 0 degrees. According to the law of reflection, the angle of reflection is also 0 degrees. So the reflected ray goes back along the same path.

In the second case, the mirror is tilted, but the light ray still falls along the normal to the tilted surface. Even though the mirror is tilted, the light ray is along the normal, so the angle of incidence is 0 degrees. Therefore, the angle of reflection is also 0 degrees, and the reflected ray goes back along the same path.

In the third case, the mirror is tilted, and the light ray falls at an angle of 20 degrees from the normal. So the angle of incidence is 20 degrees. Therefore, the angle of reflection is also 20 degrees, and the reflected ray makes an angle of 20 degrees on the other side of the normal.

Question 3: In Fig. 10.23, the cap of a sketch pen is placed in front of three types of mirrors. Match each image with the correct mirror.

Now students, think about what we've learned. A plane mirror forms an image of the same size as the object and erect. A convex mirror always forms an erect and diminished image. A concave mirror, when the object is close, forms an enlarged and erect image, but when the object is far, it forms an inverted image.

So if image (i) shows the cap of the same size and erect, it must be a plane mirror. If image (ii) shows the cap smaller than the object and always erect, it must be a convex mirror. If image (iii) shows the cap larger than the object or inverted depending on distance, it must be a concave mirror.

So the matching is: (i) with Plane mirror, (ii) with Convex mirror, (iii) with Concave mirror.

Question 4: In Fig. 10.24, the cap of a sketch pen is placed behind a convex lens, a concave lens, and a flat transparent glass piece — all at the same distance. Match each image with the correct type of lens or glass.

Now students, think about what we've learned. A flat transparent glass piece is like a plane mirror for light passing through — it doesn't change the size or orientation of the image. A convex lens, when the object is close, forms an enlarged and erect image, but when the object is far, it forms an inverted image. A concave lens always forms an erect and diminished image.

So if image (i) shows the cap of the same size and erect, it must be the flat transparent glass piece. If image (ii) shows the cap larger than the object or inverted depending on distance, it must be a convex lens. If image (iii) shows the cap smaller than the object and always erect, it must be a concave lens.

So the matching is: (i) with Flat transparent glass piece, (ii) with Convex lens, (iii) with Concave lens.

Question 5: When the light is incident along the normal on the mirror, which of the following statements is true?

Now students, when light is incident along the normal, it means the light ray is coming directly toward the mirror along the perpendicular line. In this case, the angle of incidence is 0 degrees, not 90 degrees. The angle of reflection is also 0 degrees. And reflection definitely takes place — the light bounces back along the same path. So the correct statement is (ii) Angle of incidence is 0 degrees.

Question 6: Three mirrors — plane, concave, and convex — are placed in Fig. 10.25. On the basis of the images of a graph sheet formed in the mirrors, identify the mirrors and write their names above the mirrors.

Students, think about what each mirror would show. A plane mirror would show the graph sheet with squares of the same size, erect, and with lateral inversion. A convex mirror would show the graph sheet with smaller squares, erect. A concave mirror would show the graph sheet with larger squares when close, or inverted squares when far.

So you would identify each mirror based on the characteristics of the image of the graph sheet.

Question 7: In a museum, a woman walks towards a large concave mirror. She will see that:

Now students, think about what happens when an object approaches a concave mirror. When the object is far from a concave mirror, the image is inverted and diminished. As the object comes closer, the image becomes inverted but larger. When the object reaches the focal point, the image goes to infinity and is inverted and highly magnified. When the object comes closer than the focal point, the image becomes virtual, erect, and magnified. So as she walks towards the concave mirror, her inverted image keeps increasing in size and eventually it becomes erect and magnified when she passes the focal point. So the correct answer is (iii) her inverted image keeps increasing in size and eventually it becomes erect and magnified.

Question 8: Hold a magnifying glass over text and identify the distance where you can see the text bigger than they are written. Now move it away from the text. What do you notice? Which type of lens is a magnifying glass?

Students, a magnifying glass is a convex lens. When you hold it at the right distance — closer than the focal point — you see an enlarged, erect image. When you move it away beyond the focal point, the image becomes inverted and then smaller. So you notice that the image changes from enlarged and erect to inverted and diminished as you move the magnifying glass away.

Question 9: Match the entries in Column I with those in Column II.

Column I has: (i) Concave mirror, (ii) Convex mirror, (iii) Convex lens, (iv) Concave lens.

Column II has: (a) Spherical mirror with a reflecting surface that curves inwards, (b) It forms an image which is always erect and diminished in size, (c) Object placed behind it may appear inverted at some distance, (d) Object placed behind it always appears diminished in size.

Now let's match them:

(i) Concave mirror — This is a spherical mirror with a reflecting surface that curves inwards. So (i) matches with (a).

(ii) Convex mirror — It forms an image which is always erect and diminished in size. So (ii) matches with (b).

(iii) Convex lens — Object placed behind it may appear inverted at some distance. When the object is far from a convex lens, the image is inverted. So (iii) matches with (c).

(iv) Concave lens — Object placed behind it always appears diminished in size. A concave lens always forms an erect and diminished image. So (iv) matches with (d).

Question 10: The following question is based on Assertion/Reason.

Assertion: Convex mirrors are preferred for observing the traffic behind us.

Reason: Convex mirrors provide a significantly larger view area than plane mirrors.

Now students, both the assertion and the reason are correct. Convex mirrors are indeed preferred for side-view mirrors because they provide a wider field of view. The reason correctly explains why — because convex mirrors diverge light and therefore cover a larger area. So the correct option is (i) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.

Question 11: In Fig. 10.27, note that O stands for object, M for mirror, and I for image. Which of the following statements is true?

Now students, you need to look at the figure to determine what type of mirror is shown. If the image is erect and smaller than the object, it's a convex mirror. If the image is inverted and larger than the object, it's a concave mirror. If the image is the same size and erect, it's a plane mirror. Look at the figure and determine which mirror is shown in each case.

Question 12: Place a pencil behind a transparent glass tumbler. Now fill the tumbler halfway with water. How does the pencil appear when viewed through the water? Explain why its shape appears changed.

Now students, this is a case of refraction, which we will study in detail in higher classes. When light passes from water to air, it bends or refracts. The pencil appears bent or broken at the water line because the light rays coming from the part of the pencil underwater change direction when they exit the water. This makes the underwater part appear in a different position than it actually is, giving the illusion that the pencil is bent or broken.

Now, students, let's look at the "Discover, design, and debate" section.

First, visit a nearby hospital or the clinic of an ENT specialist or a dentist, with your teacher or parents. Request the doctor to show you the mirrors used for examining ear, nose, throat, and teeth. Identify the kind of mirror used in these instruments. Students, these doctors use concave mirrors because they provide an enlarged view of the area being examined, which helps them see small problems clearly.

Second, harnessing sunlight is key to solving future energy challenges. In devices like solar cookers, mirrors are used to converge sunlight and generate heat. In India, such designs are used in villages, thus saving electricity and reducing fossil fuel use. Think of a design for a solar cooker for your school or home and prepare a detailed proposal for it including the budget required. Students, you can design a solar cooker using concave mirrors to focus sunlight onto a cooking pot. The mirror concentrates the heat, which can be used to cook food. This is an eco-friendly way to cook in sunny areas.

Third, use online tools or animation to do virtual experiments with spherical mirrors and lenses. Move objects in the simulation and observe how the image changes. This will help you visualize how the position of the object affects the image formed by mirrors and lenses.

Now, students, let's learn about our scientific heritage. More than 800 years ago, during the time of the great Indian mathematician Bhaskara II, astronomers used shallow bowls of water to observe the stars and planets. By carefully looking at their reflected images through tubes placed at appropriate angles, they could measure the positions of stars and planets in the sky. Even though the laws of reflection are not mentioned in literature, their instruments and methods indicate that they might have understood it in practice! This is amazing, isn't it? Indian scientists were doing such advanced work hundreds of years ago.

Now, students, I want you to reflect on the questions framed by your friends and try to answer them. Discuss with your classmates and see if you can answer each other's questions about light, mirrors, and lenses.

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

In this chapter on Light: Mirrors and Lenses, we started with the story of Meena who saw unusual images in curved mirrors at a science centre. We learned that mirrors with curved reflecting surfaces are called spherical mirrors, and they are of two types: concave mirrors, which curve inward, and convex mirrors, which curve outward.

We performed activities to understand the characteristics of images formed by these mirrors. We found that a concave mirror can form images that are enlarged or diminished, erect or inverted, depending on the distance of the object from the mirror. A convex mirror always forms an erect and diminished image.

We then studied the laws of reflection. The first law states that the angle of incidence equals the angle of reflection. The second law states that the incident ray, the normal, and the reflected ray all lie in the same plane. These laws apply to all types of mirrors.

We learned that concave mirrors converge parallel light rays to a point called the focal point, while convex mirrors diverge them. This property has many practical applications — concave mirrors are used in torch reflectors, headlights, and by dentists, while convex mirrors are used as side-view mirrors, road safety mirrors, and surveillance mirrors in stores.

We also learned that concave mirrors can focus sunlight to a point and generate enough heat to burn paper, which is the principle behind solar concentrators and solar furnaces.

Then we moved on to lenses. We learned that a lens is a piece of transparent material with curved surfaces. Convex lenses are thicker at the middle and converge light rays, while concave lenses are thicker at the edges and diverge light rays.

We performed activities to understand image formation by lenses. A convex lens can form images that are enlarged or diminished, erect or inverted, depending on the object's distance. A concave lens always forms an erect and diminished image.

We learned that convex lenses are used in eyeglasses, cameras, telescopes, microscopes, and even in our own eyes. The lens in our eye can change shape to focus on objects at different distances — this is called accommodation.

We solved various questions to test our understanding, including questions about reflection, image formation by mirrors and lenses, and practical applications.

We also learned about the historical use of reflective surfaces in ancient India, where astronomers used bowls of water to observe stars and planets.

Students, this chapter has taught us how mirrors and lenses work, how they form images, and how they are used in our daily lives. Understanding these concepts is not only important for your exams but also helps you appreciate the science behind many things we see and use every day.

So that's it for today, students. Keep curious, keep exploring, and remember that science is all around us! Thank you for listening attentively. See you in the next lesson.