Question

An object is placed at a distance of 20 cm in front of a concave mirror of focal length 10 cm. Calculate the image distance (in cm).

• A. -20

Answer 1

The correct option is A -20

Using the sign conventions for spherical mirrors,
Object distance, u = -20 cm
Focal length, f = -10 cm

Using the mirror formula,

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

$\frac{1}{-10}=\frac{1}{v}+\frac{1}{-20}$

$\Rightarrow$ v = -20