Question

A man can see objects clearly up to $3m$. What type of lens he should use in order to see clearly up to $12m$?

• A. Convex lens, $f=4m$
• B. Concave lens, $f=4m$
• C. Convex lens, $f=-4m$
• D. Concave lens, $f=-4m$

Answer 1

The correct option is D Concave lens, $f=-4m$

Clearly, the lens used should be the concave lens.

Actual far point

$u=-12m$

Far point of the person,

$v=-3m$

We have lens formula,

$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

$\Rightarrow \frac{1}{-3}+\frac{1}{12}=\frac{1}{f}$

$\Rightarrow f=-4m$

Thus a concave lens of focal length $4m$ is used.