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A person needs a lens of power –5.5 diopters for correcting his distant vision. For correcting his near vision, he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting :
(i) distant vision, and (ii) near vision?
(i) for far sight :
P = – 5.5 D
Power of lens = 1/f (in m)
∴ f = 1/P
f = 1/(-5.5)
f = -10/15
f = – 0.182 = – 0.18 m
(ii) for near sight
P = + 1.5
∴ P = 1/f (in m)
∴ f = 1/P
f = 1/1.5
f = 10/15
f = 2/3
f = 0.66 cm = 0.67 cm
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