A concave mirror gives an image three times as large as the object placed at a distance of 20 cm from it . For the image to be real , the focal length should be-

Options

A. `10` cm
B. `15` cm
C. `20` cm
D. `30` cm

Answer

B

Explanation

u=`-20` cm, v=mu=`-60` cm
`(1)/(f)=(1)/(v)+(1)/(u)=(-1)/(60)-(1)/(20)=(-4)/(60)=(-1)/(15),f=-15` cm