Question

A concave mirror produces an image of an object such that the distance between the object and image is 20 cm . If the magnification of the image is ' -3 ', then the magnitude of the radius of curvature of the mirror is :

(1) 3.75 cm

(2) 30 cm

(3) 7.5 cm

(4) 15 cm

Answer 1

Correct option is (4) 15 cm  

\(\mathrm{m}=-3=-\frac{\mathrm{v}}{\mathrm{u}}\) and \(\mathrm{v}-\mathrm{u}=20 \mathrm{~cm}\)

\(f=\frac{v u}{v+u}=\frac{(-30)(-10)}{-30-10}\)

\(\therefore \mathrm{R}=+15\)