Question

A long-sighted person cannot see objects clearly at a distance less than $40cm$ from his eye. The power of the lens needed to read an object at $25cm$ is

• A. $-2.5D$
• B. $+2.5D$
• C. $-6.25D$
• D. $+1.5D.$

Answer 1

The correct option is D $+1.5D.$

So, image of object at $25cm$ should be created at $40cm$
$u=25cm,v=-40cm$
$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}=\frac{-1}{40}+\frac{1}{25}=\frac{-8+5}{200}$
$=\frac{-5+8}{200}$
$\Rightarrow f=\frac{200}{3}cm=\frac{2}{3}m$
$P=\frac{1}{f}=+1.5D$.