Question

An object of height $1.5cm$ is situated at a distance of $15cm$ from a concave mirror. The concave mirror forms its real image of height $3.0cm$. The focal length of concave mirror will be :

• A. $-10cm$
• B. $-20cm$
• C. $20cm$
• D. $30cm$

Answer 1

The correct option is A $-10cm$

Given,

object height $h_1=1.5cm$

real image length $h_2=-3cm$

Position of object from the mirror $u=-15cm$

Mirror magnification formula

$m=\frac{h_2}{h_1}=-\frac{v}{u}$

$\frac{-3}{1.5}=\frac{-v}{-15}$

$v=-30cm$

Apply Mirror formula

$\frac{1}{f}=\frac{1}{-30}+\frac{1}{-15}$

$f=-10cm$