Radii of curvature of a converging lens are in the ratio `1 : 2`. Its focal length is `6 cm` and refractive index is `1.5`. Then its radii of curvature are

Options

A. `9 cm` and `18 cm`
B. `6 cm` and `12 cm`
C. `3 cm` and `6 cm`
D. `4.5 cm` and `9 cm`

Answer

D

Explanation

Here, `f = 6 cm. mu = 1.5, R_(1) = R, R_(2) = -2R`
According to lens maker's formula
`:. 1/6 = (1.5-1) (1/R - (1)/(-2R)) = 0.5((1)/(R) + (1)/(2B))`
`R = (1.5 xx 6)/(2) = 4.5 cm`
`:. R_(1) = 4.5 cm, R_(2) = 9 cm`