Question

A person can see clearly only up to a distance of 25 cm. He wants to read a book placed at a distance of 50 cm. What kind of lens does he require for his spectacles and what must be its power ?

• A. Concave, -2.0 D
• B. Convex, +2.0 D
• C. Concave, -1.0 D
• D. Convex, +1.5 D

Answer 1

The correct option is A Concave, -2.0 D

We know that: $\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$

Person can see distance of 25 cm clearly and he wants to read book at 50 cm, so he will use a lens that can make image of 50 cm distance at 25 cm, therefore,

$u=-50cmandv=-25cm$


Now applying lens formula we will get f,

$\Rightarrow \frac{1}{f}=\frac{1}{-25}-\frac{1}{-50}=-\frac{1}{50}$

$\Rightarrow f=-50cm$

And power is given by:
$P=1/f=-100/50=-2D$

Negative sign of focal length means concave lens is used.

Final answer: (C)