Question

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

• A. +½
• B. -½
• C. +1
• D. -1

Answer 1

The correct option is D -1

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

From the mirror formula,
$\frac{1}{f}=\frac{1}{v}+\frac{1}{u}$
$\frac{1}{-10}=\frac{1}{v}+\frac{1}{-20}$
$v=-20cm$

From the magnification formula,
$m=-\frac{v}{u}=\frac{h^{\prime }}{h}$
$m=\frac{-(-20)}{(-20)}=-1$