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, R_1 = R` and `R_2 = -R, f = 20 cm`
As `(1)/(f) = (mu - 1) ((1)/(R_1) - (1)/(R_2)) :. (1)/(20) = (1.55 - 1) ((1)/( R) + (1)/(R )) = (1.10)/( R)`
`R = 20 xx 1.1 = 22 cm`.