Question

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

its virtual image is formed 20 cm from the mirror? 

Answer 1

Given 

Given,f=−10 cm v=+20 cm Since the image is virtual and forms behind the mirror, the equation will be  

`1/f=1/v+1/u` 

`1/-10=1/20+1/u` 

`-1/10-1/20=1/u` 

`1/u=(-20-10)/200` 

`1/u=(-30)/200` 

`1/u=(-3)/20` 

`u=-20/3` 

Therefore, the object must be placed at a distance of `20/3`cm from the mirror to form the virtual image.