A double convex lens is made of glass of refractive index `1.55` with both faces of same radius of curvature. Find the radius of curvature required, if focal length is `20 cm`.

Explanation

Here, `mu = 1.55, f = 20 cm`
Let `R_1 = - R_2 = R`
From `(1)/(f) = (mu - 1) ((1)/(R_1) - (1)/(R_2)) (1)/(20) = (1.55 - 1) ((1)/( R) + (1)/( R)) = (0.55 xx 2)/( R)`
`R = 20 xx 0.55 xx 2 = 22 cm`.