10th Light – Reflection and Refraction NCERT Solution

Light – Reflection and Refraction

Question 1

1. Which one of the following materials cannot be used to make a lens?

(a) Water (b) Glass (c) Plastic (d) Clay

Answer 1

Answer: (d) Clay

Explanation: A lens is a transparent medium that refracts light. Clay is an opaque material that does not allow light to pass through it, so it cannot be used to make a lens.

Question 2

2. The image formed by a concave mirror is observed to be virtual, erect and larger than the object. Where should be the position of the object?

(a) Between the principal focus and the centre of curvature

(b) At the centre of curvature

(c) Beyond the centre of curvature

(d) Between the pole of the mirror and its principal focus.

Answer 2

Answer: (d) Between the pole of the mirror and its principal focus.

Explanation: When an object is placed between the pole and principal focus of a concave mirror, the image formed is virtual, erect, and larger than the object.

Question 3

3. Where should an object be placed in front of a convex lens to get a real image of the size of the object?

(a) At the principal focus of the lens

(b) At twice the focal length

(c) At infinity

(d) Between the optical centre of the lens and its principal focus.

Answer 3

Answer: (b) At twice the focal length

Explanation: When an object is placed at twice the focal length of a convex lens, the image formed is real and of the same size as the object.

Question 4

4. A spherical mirror and a thin spherical lens have each a focal length of –15 cm. The mirror and the lens are likely to be

(a) both concave.

(b) both convex.

(c) the mirror is concave and the lens is convex.

(d) the mirror is convex, but the lens is concave.

Answer 4

Answer: (a) both concave.

Explanation: A negative focal length indicates that both the mirror and lens are concave.

Question 5

5. No matter how far you stand from a mirror, your image appears erect. The mirror is likely to be

(a) only plane.

(b) only concave.

(c) only convex.

(d) either plane or convex.

Answer 5

Answer: (d) either plane or convex.

Explanation: Both plane mirrors and convex mirrors always form erect images, regardless of the object's distance.

Question 6

6. Which of the following lenses would you prefer to use while reading small letters found in a dictionary?

(a) A convex lens of focal length 50 cm.

(b) A concave lens of focal length 50 cm.

(c) A convex lens of focal length 5 cm.

(d) A concave lens of focal length 5 cm.

Answer 6

Answer: (c) A convex lens of focal length 5 cm.

Explanation: A convex lens with a shorter focal length will provide greater magnification, making it easier to read small letters.

Question 7

7. We wish to obtain an erect image of an object, using a concave mirror of focal length 15 cm. What should be the range of distance of the object from the mirror? What is the nature of the image? Is the image larger or smaller than the object?

Answer 7

Answer: The object should be placed between 0 cm and 15 cm from the mirror. The image is virtual, erect, and larger than the object.

Question 8

8. Name the type of mirror used in the following situations.

(a) Headlights of a car.

(b) Side/rear-view mirror of a vehicle.

(c) Solar furnace.

Support your answer with reason.

Answer 8

Answer:

(a) Concave mirror: used in headlights to converge light rays and produce a parallel beam.

(b) Convex mirror: used in side/rear-view mirrors to provide a wide field of view and erect image.

(c) Concave mirror: used in solar furnaces to converge sunlight and produce heat.

Question 9

9. One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object?

Answer 9

Answer: Yes, the lens will produce a complete image of the object.

Explanation: Even with half the lens covered, the remaining portion will still refract light and form a complete image.

Question 10

10. An object 5 cm in length is held 25 cm away from a converging lens of focal length 10 cm. Draw the ray diagram and find the position, size and the nature of the image formed.

Answer 10

Step-by-Step Solution:

Given: Object height (h) = 5 cm, Object distance (u) = -25 cm, Focal length (f) = 10 cm

1. Find the image distance (v) using the lens formula: 1/f = 1/v - 1/u

...

Answer: Image position: 16.67 cm (behind the lens), Image size: 3.33 cm, Nature: Real and inverted.

Question 11

11. A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens?

Answer 11

Step-by-Step Solution:

Given: Focal length (f) = -15 cm, Image distance (v) = -10 cm

1. Find the object distance (u) using the lens formula: 1/f = 1/v - 1/u

...

Answer: Object distance: 30 cm.

Question 12

12. An object is placed at a distance of 10 cm from a convex mirror of focal length 15 cm. Find the position and nature of the image.

Answer 12

Step-by-Step Solution:

Given: Object distance (u) = -10 cm, Focal length (f) = 15 cm

1. Find the image distance (v) using the mirror formula: 1/f = 1/v + 1/u

...

Answer: Image position: 6 cm (behind the mirror), Nature: Virtual and erect.

Question 13

13. The magnification produced by a plane mirror is +1. What does this mean?

Answer 13

Answer: The image is virtual, erect, and of the same size as the object.

Question 14

14. An object 5.0 cm in length is placed at a distance of 20 cm in front of a convex mirror of radius of curvature 30 cm. Find the position of the image, its nature and size.

Answer 14

Step-by-Step Solution:

Given: Object height (h) = 5 cm, Object distance (u) = -20 cm, Radius of curvature (R) = 30 cm

...

Answer: Image position: 8.57 cm (behind the mirror), Nature: Virtual and erect, Size: 2.14 cm.

Question 15

15. An object of size 7.0 cm is placed at 27 cm in front of a concave mirror of focal length 18 cm. At what distance from the mirror should a screen be placed, so that a sharp focussed image can be obtained? Find the size and the nature of the image.

Answer 15

Step-by-Step Solution:

Given: Object height (h) = 7 cm, Object distance (u) = -27 cm, Focal length (f) = -18 cm

...

Answer: Screen distance: 54 cm, Image size: 14 cm, Nature: Real and inverted.

Question 16

16. Find the focal length of a lens of power –2.0 D. What type of lens is this?

Answer 16

Step-by-Step Solution:

Given: Power (P) = -2.0 D

1. Find the focal length (f): f = 1/P = 1/(-2.0) = -0.5 m

Answer: Focal length: -0.5 m, Type: Concave lens.

Question 17

17. A doctor has prescribed a corrective lens of power +1.5 D. Find the focal length of the lens. Is the prescribed lens diverging or converging?

Answer 17

Step-by-Step Solution:

Given: Power (P) = +1.5 D

1. Find the focal length (f): f = 1/P = 1/(+1.5) = 0.67 m

Answer: Focal length: 0.67 m, Type: Converging lens (Convex lens).

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