Question

The far point of a myopic person is 80 cm in front of the eye. 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 him, $v=-80cm$ (far point), $u=\infty$
So $\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
$\Rightarrow \frac{1}{f}=\frac{1}{v}\left(as\frac{1}{\infty =0}\right)\Rightarrow \frac{1}{f}=\frac{1}{-80}\Rightarrow f=-80cm\Rightarrow f=-0.8m\therefore P=\frac{1}{-0.8}=-1.25D\left(Dioptres\right)$
where p is the power of the lens, f is the focal length, v is the image distance and u is the object distance.