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`.

Options

A. `11 cm`
B. `22 cm`
C. `7 cm`
D. `6 cm`

Answer

B

Explanation

Lens maker's formula `1/f = (mu-1) ((1)/(R_(1)) - (1)/(R_(2)))`
Here, `f = 20 cm , mu = 1.55, R_(1) = R` and `R_(2) = -R`
`:. 1/20 = (1.55 - 1) ((1)/(R) - ( 1)/(-R))` or `1/20 = 0.55 xx 2/R`
`rArr R = 1.1 xx 20 = 22 cm`