Question

An object is placed on the principal axis of a concave mirror at a distance of $60cm$. If the focal length of the concave mirror is $40cm$ then determine the magnification of the obtained image.

• A. $4$
• B. $-2$
• C. $-4$
• D. $+2$

Answer 1

The correct option is B $-2$

Given:

Distance of object $u=-60cm$

Focal length $f=-40cm$

From mirror formula
$\frac{1}{v}+\frac{1}{u}=\frac{1}{f}$
$\frac{1}{v}-\frac{1}{60}=\frac{-1}{40}$
$\frac{1}{v}=\frac{1}{60}-\frac{1}{40}$
$v=-120cm$
Hence magnification is given by
$m=\frac{-v}{u}=-\frac{(-120)}{(-60)}$
$m=-2$