Question

A concave mirror produces three times enlarged image of an object placed at 10 cm in front of it. Calculate the focal length of the mirror.

Answer 1

Given,

Magnification, $m=3$

Object distance, $u=10cm$

We know,

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

$3=\frac{-v}{-10}$

$\Rightarrow$ $v=30cm$

From mirror formula,

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

$\frac{1}{f}=\frac{1}{30}-\frac{1}{10}$

$=\frac{10-30}{30\times 10}$

$=\frac{-20}{300}$

$f=-15cm$

Focal length, $f=-15cm$