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**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 â€“ Convex lens.

b) Negative power â€“ 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, â€“ 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, â€“ 4 D ?**

**Solution : **Concave lens.

**Question 18: The opticianâ€™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.

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