Question

If the far point of a myopic person is $75cm$, what should be the focal length of the lens used to rectify this defect?

Answer 1

Given: the far point of a myopic person is $75cm$

To find: the focal length of the lens used to rectify this defect

Solution:

The person suffering from myopia should use a concave lens such that the image of an object at infinity is formed at a distance of $75cm$

i.e

$u=\infty$

$v=-75cm$

Applying lens formula,

$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}⟹\frac{1}{f}=\frac{1}{-75}-\frac{1}{\infty }⟹f=-75cm$

is the required focal length of the lens to be used to rectify the defect.