The radii of curvature of the two surface of a lens are 20 cm and 30 cm and the refractive index of the material of the lens is 1.5 . If the lens is concave - convex, then the focal length of the lens is :

Options

A. 24 cm
B. 10 cm
C. 15 cm
D. 120 cm

Answer

D

Explanation

The focal length of the lens,
`1/f = (mu - 1) (1/(R_1) - 1/(R_2)) implies 1/f = (1.5 - 1)(1/20 - 1/30)`
`implies 1/f = 0.5 ((30- 20)/(600))`
`implies f = 120 cm`