Question

An object is placed at 20 cm in front of a concave mirror produces three times magnified real image. What is focal length of the concave mirror?

• A. 15 cm
• B. 6.6 cm
• C. 10 cm
• D. 7.5 cm

Answer 1

The correct option is A 15 cm

Given : Object distance, $u=-20$ cm

A concave mirror forms a real magnified image only when the image to be formed is inverted i.e. $m=-3$

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

$\therefore$ $-3=\frac{-v}{-20}$ $⟹v=-60$ cm

Using mirror formula : $\frac{1}{v}+\frac{1}{u}=\frac{1}{f}$

$\therefore$ $\frac{1}{-60}+\frac{1}{-20}=\frac{1}{f}$

$⟹$ $f=\frac{-60}{4}=-15$ cm

Thus focal length of the mirror is 15 cm.