A person needs a lens of power -5.5 dioptre 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?

The power P of a lens of focal length f is given by the relation P= 1/f
(i) Power of the lens used for correcting distant vision = – 5.5 D
Focal length of the required lens, f= 1/Pf= 1/-5.5 = -0.181 m
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 = +0.667 m
The focal length of the lens for correcting near vision is 0.667 m

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