Find the focal length of a concave mirror if it produces two times large image of an object placed at 20cm from it

 

Solution:-

Object distance(u)=-20 cm

Magnification(m)=2

-v/u=2

-v/20=2

-V=40

v=-40 cm

From mirror formula:-

1/f=1/v+1/u

     =(1/-40)+(1/-20) 

    =-(1/40+1/20)

    =-(1+2)/40

1/f=-3/40

f=-40/3

f=-13.33 cm