Question

A man can see the object up to a distance of one meter from his eyes. For correcting his eyesight so that he can see an object at infinity, he requires a lens whose power is

• A. $+0.5D$
• B. $+1.0D$
• C. $+2.0D$
• D. $-1.0D$

Answer 1

The correct option is D $-1.0D$

The image of object at infinity should be formed at $100cm$ from the eye

Apply formula:

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

$f$ : focal length

$u$ : object distance

$v$ : Image distance
$\frac{1}{f}=\frac{1}{\infty }-\frac{1}{100}$

$\Rightarrow \frac{1}{f}=-\frac{1}{100}$

Apply formula of power:

$P=\frac{1}{f}$
So the power $=\frac{-100}{100}=-1D$
( Distance is given in $cm$ but $P=\frac{1}{f}$ in metres )