The radii of curvature of the two surfaces 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

(d) The focal length of the lens,
`1/f=(mu-1)(1/R_(1)-1/R_(2))`
`1/f=(1.5-1)(1/20-1/30)rArr1/f=0.5((30-20)/600)`
`or1/f=1/2xx10/600=1/120`
or The focal length, f = 120 cm