NCERT Class 8 Science Curiosity Chapter 10 Light Mirrors and Lenses PDF Download

Chapter 10: Light: Mirrors and Lenses

Probe and ponder

• Can we make mirrors which can give enlarged or diminished images?
• On side-view mirrors of vehicles, there is a warning that says "Objects in mirror are closer than they appear". Why is this warning written there?
• Why is there a curved line on some reading glasses?
• Share your questions

During the summer holidays, Meena went to a science centre with her family. The centre had many fascinating displays on nature, space, and technology. While her parents explored 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 puzzled.

She remembered doing activities with a mirror earlier where the image formed by the mirror was of the same size as the object and was erect (in the chapter 'Light: Shadows and Reflections' in Curiosity, Grade 7). Seeing her confusion, a guide from the science centre walked up to her and smiled. "These are not plane mirrors," the guide explained. "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 talk to her teacher about these spherical mirrors.

10.1 What Are Spherical Mirrors?

Activity 10.1: Let us explore

• Take a shiny metallic spoon and hold its curved surface close to your face. Can you see your image in it?
• Notice the image of your face. Is it different from the image you see in a plane mirror?
• While observing the image, slowly move the spoon away from your face. Do you observe any change in the image?
• Now flip the spoon and repeat the same steps.

Did you notice that the shiny metallic spoon acted like a mirror and you could see your image in it? When you looked at the inner side of the spoon which is curved inwards, you must have observed that the image was inverted (Fig. 10.1a). When you looked at the outer side of the spoon which bulges outwards, the image of your face was erect but smaller in size (Fig. 10.1b).

[Figure 10.1: Image formed in a shiny metallic spoon from its (a) Inner curved surface; (b) Outer curved surface, See in your textbook]

Curved mirrors, like the spoon, can also be specially made. Spherical mirrors are a common type of curved mirrors which are shaped like a part of a hollow glass sphere. Mirrors, whose reflecting surfaces are spherical are called spherical mirrors.

Teacher's Note

You will encounter both concave and convex mirrors in this chapter, and they behave very differently. A quick memory trick: in a concave mirror, the reflecting surface curves inward like a "cave" (notice the word "cave" in "concave"). In a convex mirror, the surface curves outward. This helps you remember which type creates which type of image.

The reflecting surface of the spherical mirror may be curved inwards or outwards. A spherical mirror, which has a reflecting surface that curves inwards, is called a concave mirror (Fig. 10.2a). Its schematic representation is shown in Fig. 10.2b. The outline of the surface of the mirror is circular.

[Figure 10.2: (a) Concave mirror; (b) Its representation, See in your textbook]

A spherical mirror which has a reflecting surface that curves outwards is called a convex mirror (Fig. 10.3a). Its schematic representation is shown in Fig. 10.3b.

[Figure 10.3: (a) Convex mirror; (b) Its representation, See in your textbook]

In the representation of both the mirrors, the non-reflecting surface of the mirror is shown as shaded.

A step further

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.

[Figure: Diagram showing an imaginary hollow sphere, a piece of imaginary hollow sphere becoming a concave mirror and a convex mirror with labeled coated and reflecting surfaces, See in your textbook]

Teacher's Note

When you describe where the reflecting coating is placed, be precise: the coating on the outside of the curve makes a concave mirror (light bounces inward), and the coating on the inside makes a convex mirror (light bounces outward). Exam questions sometimes ask you to explain which surface gets the reflective coating, so learning this now will help you answer correctly.

Key Points

• Spherical mirrors are curved mirrors shaped like part of a hollow glass sphere, unlike plane mirrors which are flat.
• A concave mirror curves inward and can produce enlarged, reduced, or inverted images depending on where the object is placed.
• A convex mirror curves outward and always produces a smaller, erect, and virtual image.
• Spherical mirrors are made by applying a reflective coating to a curved glass surface; the location of the coating determines whether the mirror is concave or convex.

This is a preview of the first 3 pages. To get the complete chapter, click below.