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
near vision?

Explanation

The power P of lens of focal length f is given by the relation P=1/f
Power of the lens used for correcting distant vision=-5.5D
Focal length of the required lens, f=1/Pf=i1/-5.5=-0.181m
The focal length of the lens for correccting distant vision is -0.181m