Lakhmir Singh Physics Class 10 Solutions Page No:261
Question 1: The lens A has a focal length of 25 cm whereas another lens B has a focal length of 60 cm. Giving reason state, which lens has more power : A or B
Solution : Lens A has more power because of its shorter focal length.
Question 2: Which causes more bending (or more refraction) of light rays passing through it: a convex lens of long focal length or a convex lens of short focal length ?
Solution : Convex lens of short focal length causes more bending of light rays passing through it.
Question 3: Name the physical quantity whose unit is dioptre.
Solution : Power of a lens.
Question 4: Define 1 dioptre power of a lens.
Solution : 1 diopter is the power of a lens whose focal length is 1 metre.
Question 5: Which typie of lens has (a) a positive power, and (b) a negative power ?
Solution : a) Positive power – Convex lens.
b) Negative power – Concave lens.
Question 6: Which of the two has a greater power : a lens of short focal length or a lens of large focal length ?
Solution : Lens of short focal length.
Question 7: How is the power of a lens related to its focal length ?
Solution : Power of lens is reciprocal of its focal length in metres.
Question 8: Which has more power : a thick convex lens or a thin convex lens, made of the same glass ? Give reason for your choice.
Solution : Thick convex lens has more power because of its shorter focal length.
Question 9: The focal length of a convex lens is 25 cm. What is its power ?
Solution : f = 25 cm = 0.25 cm.
P = 1/f = 1/0.25 = +4 D.
Question 10: What is the power of a convex lens of focal length 0.5 m ?
Solution : f = 0.5 m
P = 1/f = 1/0.5 = +2 D.
Question 11: A converging lens has a focal length of 50 mm. What is the power of the lens ?
Solution : f = 50 mm = 0.05 m
P = 1/f = 1/0.05 = +20 D.
Question 12: What is the power of a convex lens whose focal length is 80 cm ?
Solution : f = 80 cm = 0.8 m.
P = 1/f = 1/0.8 = +1.25 D.
Question 13: A diverging lens has a focal length of 3 cm. Calculate the power.
Solution : Here, f = -3 cm = -0.03 m (Diverging lens)
P = 1/f = 1/(-0.03) = -33.33 D.
Question 14: The power of a lens is + 0.2 D. Calculate its focal length.
Solution : P = +0.2 D.
P = 1/f.
f = 1/P = 1/0.2 = +5 m.
Question 15: The power of a lens is, – 2D. What is its focal length ?
Solution : P = -2 D.
P = 1/f.
f = 1/P = 1/(-2) = -0.5 m = -50 cm.
Question 16: What is the nature of a lens having a power of + 0.5 D ?
Solution : Convex lens.
Question 17: What is the nature of a lens whose power is, – 4 D ?
Solution : Concave lens.
Question 18: The optician’s prescription for a spectacle lens is marked + 0.5 D. What is the :
(a) nature of spectacle lens ?
(b) focal length of spectacle lens ?
Solution : (a) Convex lens
(b) P = +0.5 D.
P = 1/f.
f = 1/P = 1/0.5 = 2 m.
Question 19: 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 ?
Solution : P = -1.5 D
P = 1/f.
f = 1/P = 1/(-0.5) = -0.66 m = -66.6 m.
Since focal length is negative, it is a diverging lens.