Question

A man cannot see an object closer than $1\text{m}$ from the eye clearly. What is the power of the corrective lens, he should use?

• A. $+3\text{D}$
• B. $-3\text{D}$
• C. $+2\text{D}$
• D. $-2\text{D}$

Answer 1

The correct option is A $+3\text{D}$

The near point of the man is $1\text{m}$ from his eye.

For him to see an object placed at the normal near point $\left(25\text{cm}\right)$, its image should form at $1\text{m}$ from the eye.

So,
Object distance, $u=-0.25\text{m}$
Image distance, $v=-1\text{m}$

From lens formula,

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

$\Rightarrow \frac{1}{(-1)}-\frac{1}{(-0.25)}=\frac{1}{f}$

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

$\Rightarrow P=+3\text{D}$