Question

At what distance from a concave mirror of focal length 10 cm should an object be placed so that:  

its real image is formed 20 cm from the mirror?

Answer 1

Given 

It is a concave mirror. 

f=-10 cm 

v=-20 cm 

Since the image is real and forms in front of the mirror, the equation will be  ${\displaystyle \frac{1}{f}=\frac{1}{v}+\frac{1}{u}}$ 

${\displaystyle \frac{1}{-10}=\frac{1}{-20}+\frac{1}{u}}$ 

${\displaystyle \frac{2}{u}=\frac{1}{-10}+\frac{1}{20}}$ 

${\displaystyle \frac{1}{u}=\frac{-20+10}{200}}$ 

${\displaystyle \frac{1}{u}=\frac{-10}{200}}$ 

${\displaystyle \frac{1}{u}=-\frac{1}{20}}$

 u=-20 cm 

Therefore, the object should be placed at a distance of 20 cm from the mirror to form a real image.