Diameter of plano-convex lens is 6 cm and thickness at the centre is 3mm. If speed of light in material of lens is 2 x 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

`R = sqrt ((R - 0.3) ^(2) + 3 ^(2))`
`R ^(2) = R ^(2) - 0.6 R + 0.09 + 9`
`R = ( 9.09)/( 0.60) cm = ( 909)/(60) = 15.15 cm`
`mu _(1) = ( 3 xx 10 ^(8))/( 2 xx 10 ^(8)) = 1. 5`
`(1)/(f) = ((mu _(L ))/( mu _(m)) -1) ( (1)/( R _(1)) - (1)/( R _(2)))`
`( 1)/(f) = ((3)/(2) - 1) ((1)/(oo) - (1)/( - 15.5))`
`(1)/(f) = (1)/(2) xx (1)/(15.15)`
`f = 30.3 cm`