Diameter of a plano-convex lens is 6 cm and thickness at the centre is 3 mm. If speed of light in material of lens is `2 xx 10^8` m/s., the focal length of the lens is ......

Options

A. 15 cm
B. 20 cm
C. 30 cm
D. 10 cm

Answer

C

Explanation

From irignometry

`R^(2) = (3)^(2) + (R-0.3)^(2) implies R = 15 cm`
`mu_("lens") = (3 xx 10^(8))/(2 xx 10^(8)) = 1.5`
`therefore` from Lens maker formula ,
`(1)/(f) = ((mu_(L))/(mu_(s)) - 1) ((1)/(R_(1)) - (1)/(R_(2))) = 0.5 ((1)/(15)) = (1)/(30)`
f = 30 cm