Class 8 Science Chapter 10 Light: Mirrors and Lenses Questions and Answers New Syllabus Updated NCERT Solution
Keep the curiosity alive Page no: 166
Question: 1. A light ray is incident on a mirror and gets reflected by it (Fig. 10.21). The angle made by the incident ray with the normal to the mirror is 40°. What is the angle made by the reflected ray with the mirror?
(i) 40° (ii) 50° (iii) 45° (iv) 60°
Answer: The correct answer is (ii) 50°.
Explanation:-
In the scenario described, when a light ray strikes a mirror, the angle of incidence is equal to the angle of reflection. This is a fundamental principle known as the law of reflection.
Given that the angle made by the incident ray with the normal to the mirror is 40°, the angle made by the reflected ray with the normal will also be 40°.
To find the angle made by the reflected ray with the mirror, we need to consider that the angle of the mirror itself is 90° to the normal. Therefore, the angle made by the reflected ray with the mirror can be calculated as follows:
- Angle of the mirror = 90°
- Angle of reflection = 40°
Thus, the angle made by the reflected ray with the mirror is:
Angle with mirror=90°−Angle of reflection=90°−40°=50°
Question: 2. Fig. 10.22 shows three different situations where a light ray falls on a mirror:
Draw the reflected ray in each case (Use a ruler and protractor for accurate drawing). What is the angle of reflection in each case?
(i) The light ray falls along the normal.
Answer: Situation: The incident ray strikes the mirror at a 90° angle (along the normal).
- Reflection: Since the angle of incidence (i) is 0° (because it falls along the normal), the angle of reflection (r) is also 0°.
- Conclusion: The reflected ray will continue straight back along the same path as the incident ray.
(ii) The mirror is tilted, but the light ray still falls along the normal to the tilted surface.
Answer: Situation: The mirror is tilted, but the light ray still falls along the normal to the tilted surface.
- Reflection: Again, the angle of incidence (i) is 0°, which means the angle of reflection (r) is also 0°.
- Conclusion: The reflected ray will also follow the same path as the incident ray, but the direction will depend on the tilt of the mirror.
(iii) The mirror is tilted, and the light ray falls at an angle of 20° from the normal.
Answer: Situation: The mirror is tilted, and the light ray strikes at an angle of 20° from the normal.
- Reflection: According to the law of reflection, the angle of incidence (i) is 20°. Therefore, the angle of reflection (r) will also be 20°.
- Conclusion: The reflected ray will be at an angle of 20° from the normal on the opposite side.
Summary of Angles of Reflection
- Case 1: Angle of reflection = 0°
- Case 2: Angle of reflection = 0°
- Case 3: Angle of reflection = 20°
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.
Answer:
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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.
Answer:
Question: 5. When the light is incident along the normal on the mirror, which of the following statements is true:
(i) Angle of incidence is 90°
(ii) Angle of incidence is 0°
(iii) Angle of reflection is 90°
(iv) No reflection of light takes place in this case
Answer: (ii) Angle of incidence is 0°
Question: 6. Three mirrors — plane, concave and convex are placed in Fig. 10.25. On the basis of the images of the graph sheet formed in the mirrors, identify the mirrors and write their names above the mirrors. 167 Fig. 10.25
Answer: Label the mirrors:
1. The mirror that forms an image of the same size and orientation is the – Plane Mirror.
2. The mirror that shows an enlarged and erect image is the – Concave Mirror.
3. The mirror that produces a smaller and upright image is the – Convex Mirror.
Question: 7. In a museum, a woman walks towards a large concave mirror (Fig. 10.26). She will see that:
(i) her erect image keeps decreasing in size.
(ii) her inverted image keeps decreasing in size.
(iii) her inverted image keeps increasing in size and eventually it becomes erect and magnified.
(iv) her erect image keeps increasing in size.
Answer: (iii) her inverted image keeps increasing in size and eventually it becomes erect and magnified.
Explanation: Concave Mirrors: When an object (in this case, the woman) is placed close to a concave mirror, the image formed is initially erect and enlarged. As she moves closer to the mirror, the image becomes inverted and continues to grow in size until she reaches a certain point where it may appear 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?
Answer: When I hold a magnifying glass over text, I have noticed that there is a specific distance at which the letters appear larger than they are written. This distance is typically very close to the lens. As I move the magnifying glass away from the text, the letters will gradually appear smaller and eventually become indistinct.
Type of Lens: A magnifying glass is a convex lens. This type of lens is thicker in the middle than at the edges and is designed to converge light rays, making objects appear larger when viewed through it.
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Question: 9. Match the entries in Column I with those in Column II.
Answer: Here is the matching of the entries from Column I with those in Column II based on the characteristics of mirrors and lenses:
| 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. |
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. Choose the correct option:
(i) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
(ii) Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
(iii) Assertion is correct but Reason is incorrect.
(iv) Both Assertion and Reason are incorrect.
Answer: (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?
(i) Figure (a) indicates a plane mirror and Figure (b) indicates a concave mirror.
(ii) Figure (a) indicates a convex mirror and Figure (b) indicates a concave mirror.
(iii) Figure (a) indicates a concave mirror and Figure (b) indicates a convex mirror.
(iv) Figure (a) indicates a plane mirror and Figure (b) indicates a convex mirror.
Answer: The correct statement is (ii) Figure (a) indicates a convex mirror and Figure (b) indicates a concave mirror
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Question: 12. Place a pencil behind a transparent glass tumbler (Fig. 10.28a). Now fill the tumbler halfway with water (Fig. 10.28b). How does the pencil appear when viewed through the water? Explain why its shape appears changed.
Answer: When you place a pencil behind a transparent glass tumbler and then fill the tumbler halfway with water, the pencil will appear to be bent or displaced when viewed through the water. This optical phenomenon occurs due to the refraction of light.
Explanation of the Phenomenon:-
1. Refraction: This is the bending of light as it passes from one medium to another. In this case, light travels from air (a less dense medium) into water (a denser medium).
2. Change in Speed: As light enters the water, it slows down, causing it to change direction. This bending of light rays makes the pencil appear to be in a different position than it actually is.
3. Visual Effect: The result is that the pencil seems to be broken or bent at the surface of the water. The extent of this bending depends on the angle at which you are viewing the pencil and the curvature of the water surface.