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Question:

The least distance of vision of a longsighted person is 60 cm. By using a spectacle lens, this distance is reduced to 12 cm. The power of the lens is?

(󔼒/3)D

+2.0D

+5.0D

+(20/3)D

Solution:

Using, 1/f = 1/v - 1/u
Here, v = 120 cm, u = -14 cm.
∴ 1/f = 1/(120) - 1/(-14) = 1/120 + 1/14 = 1/15
1/f = 1/15 → f = 15 cm
Power, P = 100/15 = 20/3 D.