Question

A man cannot see objects closer than $1$ metre 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 B $+3\text{D}$

The near point for man is $1\text{m}$ from his eye.
For him to see an objects placed at the normal near point. Its image should be formed $1\text{m}$ from the eye.
Here $v=1\text{m}=-100\text{cm},u=-25\text{cm}.$
$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}=\frac{-1}{100\text{cm}}+\frac{1}{25\text{cm}}=\frac{3}{100\text{cm}}f=\frac{100}{3}\text{cm}=\frac{1}{3}\text{m}.$
Power of the lens, $P=\frac{1}{f}=\frac{1}{1/3}{\text{m}}^{-1}=+3\text{D}$ .