Question

A concave mirror of focal length 20cm produces an image twice the size of the object. If the image is real, then the distance of the object from the mirror is

• A. 20 cm
• B. 10 cm
• C. 30 cm
• D. 60 cm

Answer 1

The correct option is C 30 cm

The magnification produced by the mirror is given as
$m=-\frac{v}{u}=-2\Rightarrow v=2u$
From mirror formula
$\frac{1}{v}+\frac{1}{u}=\frac{1}{f}\frac{1}{2u}+\frac{1}{-u}=-\frac{1}{20}\frac{3}{2u}=-\frac{1}{20}u=-30cm$
Therefore, the object is at a distance of 30 cm from the mirror
Therefore, Option C is correct.