A person needs a lens of power –5.5 dioptres 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

For distant vision = −0.181 m, for near vision = 0.667 m
The power P of a lens of focal length f is given by the relation
P = 1/f(in metres)
(i) Power of the lens used for correcting distant vision = −5.5 D
Focal length of the required lens, f =1/P
F = 1/-5.5 = -0.181ₘ
The focal length of the lens for correcting distant vision is −0.181 m.
(ii) Power of the lens used for correcting near vision = +1.5 D
Focal length of the required lens, f =1/P
F = 1/1.5 = +0.667ₘ

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