Question

A real image obtained by a concave mirror is 4 times bigger than the object If the object displaced by 3 cm away from the mirror , the image size becomes 3 times the object size . Find the focal length of the mirror.

Answer 1

image distance $=u=-20cm$

$\Rightarrow :$ magnification

$m=4=-v/u$

so $v=80cm$

from mirror formula

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

$\frac{1}{f}=\frac{1}{80}-\frac{1}{20}$

$f=-0.0375cm$