Question

A person can see clearly object only when they lie between $50 \, cm$ to $400 \, cm$ 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, $+2.25D$
• B. Concave, $-0.25D$
• C. Concave, $-0.2D$
• D. Convex, $+0.15D$

Answer 1

Answer: (B)

The maximum distance of distinct vision = $400cm$ . So the image of the object at infinity is to be formed at $400cm$ .
Use lens formula
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$\frac{1}{- 400}-\frac{1}{\in fty}=\frac{1}{f}$
$P=-0.25 \, D$