A concave mirror has a magnification of 2.5 if its focal length is 20 cm find its image position.

Solution:- Given that 

 

Magnification(m)=2.5

-v/u=2.5

v=-2.5u

 focal length =-20 cm  

now,using mirror formula

1/v + 1/u = 1/f

1/-2.5u + 1/u=1/-20  

1-2.5/-2.5u=-1/20

-1.5/-2.5u =-1/20

3/5u=-1/20

u=-12 cm this is absolute ans because it is concave mirror according to sign convention