Question

A persons sees clearly at a distance of $100cm$, then power of lens used to see object at $40cm$ is

• A. $3D$
• B. $-3D$
• C. $-1.5D$
• D. $+1.5D$

Answer 1

The correct option is D $+1.5D$

$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
Given,$v=-100cm$ and $u=-40cm$
$\frac{1}{f}=-\frac{1}{100}-\frac{1}{-40}$
$\frac{1}{f}=-\frac{1}{100}+\frac{1}{40}$
$\frac{1}{f}=\frac{-1+2.5}{100}=\frac{1.5}{100}$
Hence, power of lens is
$P=\frac{100}{f}=100\times \frac{1.5}{100}=1.5D$