Question

A virtual image three times the size of the object is obtained with a concave mirror of radius of curvature $36cm$. The distance of the object from the mirror is:

• A. $5cm$
• B. $12cm$
• C. $10cm$
• D. $20cm$

Answer 1

Answer: (B)

$12cm$

It is given that,

Magnification

$m=3$

$m=\frac{-v}{u}$

$f=\frac{R}{2}=-18cm$

It is given that a virtual image three times the size of the object is obtained with a concave mirror. So, $v=-3u$

From mirror’s formula

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

$\frac{-1}{3u}+\frac{1}{u}=\frac{1}{-18}$

$u=-12cm$

The distance of the object from the mirror $u=12cm$ in front of concave mirror.