Diameter of a plano-convex lens is 6cm and thickness at the centre is 3mm. If speed of light in material of lens is `2xx10^(8)(m)/(s)`, The focal length of the lens is

Explanation

`mu=(c)/(v)=(3xx10^8)/(2xx10^8)=(3)/(2)`
`triangleABC`:
`AC^2=AB^2+BC^2`
`R^2=y^2+(R-t)^2=y^2+R^2-2Rt+t^2`
`t ,ltltR,t^2` is negligible
`R=(y^2)/(2t)=((3)^2)/(2xx0.3)=15cm`
`(1)/(f)=(mu-1)((1)/(R_1)-(1)/(R_2))`
`=((3)/(2-1))((1)/(infty)-(1)/(-15))`
`f=30cm`