Question

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

Answer 1

Correct Answer - C
`n=(3)/(2)`
`3^(2)+(R-3mm)^(2)=R^(2)`
`rArr 3^(2)+R^(2)-2R(3mm)+(3mm)^(2)=R^(2)`
`rArr R approx15 cm`
`(1)/(f)=((3)/(2)-1)((1)/(15)) rArr f=30 cm`