Question

The far point of a myopic person is 80 cm. What is the power of the lens required to enable him to see the distant objects clearly?

• A. -1.25 D
• B. +1.50 D
• C. +1 D
• D. -1.75 D

Answer 1

The correct option is A -1.25 D

A myopic person needs a concave lens for the correction of his eyesight.
For the person, image distance is equal to the far point.
$v=-80cm$
He needs to focus objects at infinity. So, object distance is:
$u=-\infty$

Let $f$ be the focal length of lens.
Using lens formula,
$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
$\frac{1}{f}=\frac{1}{-80cm}(as\frac{1}{\infty }=0)$
$f=-0.8m$

Power of lens is:
$P=\frac{1}{f}$
$P=\frac{1}{-0.8m}=-1.25D$