The radii of curvature of the surfaces of a double convex lens are `20 cm and 40 cm` respectively, and its focal length is `20 cm`. What is the refractive index of the material of the lens ?

Options

A. `5/2`
B. `4/3`
C. `5/3`
D. `4/5`

Answer

C

Explanation

Here `R_(1) = 20 cm`, `R_(2) = - 40 cm, f = 20 cm`
Using lens maker's formula we get,
`1/20 = (mu - 1) (1/20 + 1/40) , 1/20 = (mu-1)'(3)/(40) rArr mu = (5)/(3)`