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Question:

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 diopters. What is the focal length of the lens required for correcting (i) distant vision (ii) near vision?

Solution:

(i) Correcting distant vision
f = 1/P = 1/(-5.5) = -0.18 m
(ii) Correcting near vision
f = 1/P = 1/1.5 = 0.67 m