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  `1/f=1/v+1/u` 

`1/-10=1/-20+1/u` 

`2/u=1/-10+1/20` 

`1/u=(-20+10)/200` 

`1/u = (-10)/200` 

`1/u=-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.