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. 20
B. 21
C. 22
D. 19

Answer

C

Explanation

Using the relation,
`(1)/(f)=(mu-1)((1)/(R_(1))-(1)/(R_(2)))`, we get
`(1)/(20)=(1.55-1)[(1)/(R )-((1)/(-R))]`
`= 0.55[(1)/(R )+(1)/(R )]=0.55xx(2)/(R )`
or `R=1.10xx20=22 cm`