Question

Question 5
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

The power P of a lens of focal length f is given by the relation, $P=\frac{1}{f}$
(i) Power of the lens used for correcting distant vision $=-5.5D$
Focal length of the required lens, $f=\frac{1}{P}=\frac{1}{-5.5}=-0.181m$
The focal length of the lens used for correcting distant vision is $-0.181$ m.
(ii) Power of the lens used for correcting near vision $=+1.5D$.
Focal length of the required lens, $f=\frac{1}{P}$
$f=\frac{1}{1.5}=+0.667m$
The focal length of the lens used for correcting near vision is 0.667 m.