Class 8 Science · Chapter 10

10. Light: Mirrors and Lenses

Complete solutions to Keep the Curiosity Alive. CBSE / NCERT Class 8 Science (Curiosity).

Keep the Curiosity Alive

1A light ray is incident on a mirror. The angle made by the incident ray with the normal is 40°. What is the angle made by the reflected ray with the mirror?

Answer: (ii) 50°

Angle of reflection = angle of incidence = 40°
The normal is at 90° to the mirror.
Angle between reflected ray and mirror = 90° − 40° = 50°

2Fig. 10.22 shows three situations where a light ray falls on a mirror: (a) along the normal; (b) mirror tilted, ray along the normal to the tilted surface; (c) mirror tilted, ray at 20° from the normal. Draw the reflected ray in each case. What is the angle of reflection in each case?

Blue: incident ray. Green: reflected ray. Dashed line: normal.

(a) The ray falls along the normal. It is reflected back along the same path. Angle of reflection = 0°.
(b) The ray again falls along the normal to the tilted mirror. It goes back along the same path. Angle of reflection = 0°.
(c) The angle of incidence is 20°. The reflected ray goes on the other side of the normal. Angle of reflection = 20°.

3In 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.

Image	Mirror
(i) Smaller, erect image	Convex mirror
(ii) Enlarged, erect image	Concave mirror
(iii) Same-size, erect image	Plane mirror

4In 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.

Image	Lens/glass type
(i) Enlarged image	Convex lens
(ii) Diminished image	Concave lens
(iii) Same-size image	Flat transparent glass piece

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

Answer: (ii) Angle of incidence is 0°

The angle of incidence is measured from the normal. A ray along the normal makes 0° with it. It is reflected back along the same path.

6Three mirrors — plane, concave and convex — are placed in Fig. 10.25. On the basis of the images of the graph sheet, identify the mirrors.

Left mirror: Concave mirror — the squares of the graph appear enlarged and the lines curved.
Middle mirror: Plane mirror — the squares appear of the same size and the lines are straight.
Right mirror: Convex mirror — the squares appear smaller and the lines are curved.

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

Answer: (iii) her inverted image keeps increasing in size and eventually it becomes erect and magnified.

When she is far from a concave mirror, her image is inverted. As she comes closer, the inverted image becomes bigger. When she is very close to the mirror, the image becomes erect and enlarged.

8Hold a magnifying glass over text and find the distance where the text looks bigger. Now move it away from the text. What do you notice? Which type of lens is a magnifying glass?

When the magnifying glass is held close to the text, the letters look bigger and erect.

As it is moved away, the letters first become blurred. Beyond a certain distance, they appear inverted (upside down) and smaller.

A magnifying glass is a convex lens.

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

Column I	Column II
(i) Concave mirror	(a) Spherical mirror with a reflecting surface that curves inwards
(ii) Convex mirror	(b) It forms an image which is always erect and diminished in size
(iii) Convex lens	(c) Object placed behind it may appear inverted at some distance
(iv) Concave lens	(d) Object placed behind it always appears diminished in size

10Assertion: Convex mirrors are preferred for observing the traffic behind us. Reason: Convex mirrors provide a significantly larger view area than plane mirrors.

Answer: (i) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.

A convex mirror always forms an erect, diminished image. So it shows a much wider area of the traffic behind the vehicle.

11In Fig. 10.27, O stands for object, M for mirror and I for image. In (a), the image is of the same size as the object; in (b), the image is bigger. Which statement is true?

Answer: (i) Figure (a) indicates a plane mirror and Figure (b) indicates a concave mirror.

In (a), the image is erect and of the same size, so it is a plane mirror. In (b), the image is erect and enlarged. Only a concave mirror can form such an image. A convex mirror always forms a diminished image.

12Place 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.

The part of the pencil seen through the water looks thicker (enlarged). It also appears shifted, so the pencil looks broken at the water surface.

The tumbler filled with water has a curved surface. It acts like a convex lens. Light from the pencil bends when it passes through the water and glass. So that part of the pencil looks magnified and displaced.

Discover, Design, and Debate

The “Discover, design, and debate” projects are hands-on tasks. Do them with your classmates under the guidance of your teacher, and write your own observations.

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