Question

An object is placed at a distance of 10 cm from a concave mirror of focal length 20 cm. 

Calculate the image distance.

Answer 1

 Image Distance-

Distance of the object from the mirror 'u' = −10 cm
Focal length of the concave mirror 'f' = −20 cm

We have to find the distance of the image 'v'.

Mirror Formula: 

${\displaystyle \frac{1}{f}=\frac{1}{v}+\frac{1}{u}}$ 

Therefore 

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

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

${\displaystyle \frac{1}{v}=\frac{1}{20}}$ 

v=20cm 

 Thus, the distance of the image 'v' is 20 cm.