Question

A person can see clearly objects only when they lie between $50$cm and $400$cm from his eyes. In order to increase the maximum distance of distinct vision to infinity, the type and power of the correcting lens, the person has to use, will be?

• A. Convex, $+2.25$ dioptre
• B. Concave, $-0.25$ dioptre
• C. Concave, $-0.2$ dioptre
• D. Convex, $+0.15$ dioptre

Answer 1

Answer: (B)

Concave, $-0.25$ dioptre

Maximum distance of distinct vision $=400$cm. So image of object at infinity is to be formed at $400$cm

Use lens formula, $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

$\frac{1}{-400}-\frac{1}{\infty }=\frac{1}{f}$

$P=-0.25$D.