Question

A far-sighted person has his near point $50cm$, find the power of lens he should use to see at $25cm$, clearly.

• A. $+1D$
• B. $+2D$
• C. $-2D$
• D. $-1D$

Answer 1

The correct option is B $+2D$

Given: A farsighted person has his near point 50cm,

To find the power of lens he should use to see at 25cm, clearly.

Solution:

Distant objects need to be imaged at most 50 cm from the eye.

According to the given criteria,

$u=25cm,v=-50cm$

Applying lens formula, we get

$\frac{1}{f}=\frac{1}{v}+\frac{1}{u}⟹\frac{1}{f}=\frac{1}{-50}+\frac{1}{25}⟹\frac{1}{f}=\frac{-1+2}{50}⟹f=50cm=0.5m$

The power of the lens he should use is,

$P=\frac{1}{f}=\frac{1}{0.5}=+2D$