Question

A person can see clearly objects only when they lie between $50cm$ and $400cm$ 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

The correct option is B Concave, $-0.25$ dioptre

Maximum distance of distinct vision $=400cm$
So, image of object at infinity is to be formed at $400cm$ or $4m$
Using lens formula, $P=\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
$\frac{1}{-4}-\frac{1}{\infty }=\frac{1}{f}$
$P=\frac{1}{f}=-0.25$ D
–ive implies concave mirror