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

Options

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

Answer

C

Explanation

`n=(Velocity of light in vacuum)/(Velocity of light in medium)`
`n=(3)/(2)`
`3^2+(R_3mm)^2=R^2`
`implies3^2R^2-2R(3mm)+(3mm)^2=R^2`
`impliesR approx15cm`
`(1)/(f)=((3)/(2)-1)((1)/(15))impliesf=30cm`