Question

What should be the distance between an object and a concave mirror of focal length of 20 cm so that a virtual image is produced at the distance of 10 cm form the mirror ?

• A. 12 cm
• B. 6.6 cm
• C. 30 cm
• D. 25 cm

Answer 1

The correct option is B 6.6 cm

Given that focal length $=f=-20cm$ (concave mirror) and image distance $v=10cm$ (virtual image from behind the mirror)

From mirror formula,

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

$\Rightarrow \frac{1}{u}=\frac{1}{f}-\frac{1}{u}=\frac{1}{-20}-\frac{1}{10}=\frac{-1-3}{20}=\frac{-3}{20}$

$\Rightarrow u=\frac{-20}{3}cm.\simeq 6.67cm$