Question

A long sighted person has a least distance of distinct vision of 50 cm. He wants to reduce it to 25 cm. He should use a :

• A. Concave lens of focal length 50 cm
• B. Convex of focal length 25 cm
• C. Convex lens of focal length 50 cm
• D. Concave lens of focal length 25 cm

Answer 1

The correct option is C Convex lens of focal length 50 cm

Given, $u=\infty$

Image distance, $v=50cm$

From lens formula, $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$\frac{1}{50}-\frac{1}{\infty }=\frac{1}{f}$
$f=+50cm$
He should use a convex lens of focal length 50 cm.