Question

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 (i) distant vision, and (ii) near vision?

Answer 1

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

(i) Power of the lens used for correcting distant vision = −5.5 D

Focal length of the required lens, f =

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 =

The focal length of the lens for correcting near vision is 0.667 m.