Question

A person can see clearly objects lying between $25cm$ and $2m$ from his eye. His vision can be corrected by using spectacles of power:

• A. $+0.25D$
• B. $+0.5D$
• C. $-0.25D$
• D. $-0.5D$

Answer 1

Answer: (D)

$-0.5D$

Here, $u=2m$ ; object distance
$v=\infty$ ; image distance
Apply formula:

$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$f$ : focal length
$\Rightarrow \frac{1}{\infty }-\frac{1}{2}=\frac{1}{f}$
$\Rightarrow f=-2m$
Now, relation between power$\left(P\right)$ and focal length $\left(f\right)$-

$P=\frac{1}{f}$

$\Rightarrow P=\frac{1}{-2}D$

$\Rightarrow P=-0.5D$