A person wears eye glasses with a power of `-5.5 D` for distance viewing. His doctor prescribes a correction of `+ 1.5 D` for his near vision. What is the focal length of the lens used for correcting (i) distant vision and (ii) near vision?

Explanation

The power P of a lense of focal length f is given by the relation P=1/f
(i) Power of the lens used for correcting distance vision =-5.5 D
Focal length of the required lens, f=1/Pf=1/-5.5=-0.181m`
The focal length of the lens of correcting distance vision is -0.181m
(ii) Power of the lens used for correcting near vision =+1.5D
Focal length of the required lens, f=1/P
f=1/1.5=066.7m
The focal length of the lens for correcting near vision is 0. 667m