Question

The radii of curvature of the two surfaces of a lens are `20cm` and `30 cm` and the refractive index of the material of the lens is `1.5`. If the lens is concave`-` convex, then the focal length of lens is
A. `24 cm`
B. `10 cm`
C. `15 cm`
D. `120 cm`

Answer 1

Correct Answer - D
`(1)/(f)=(mu-1)((1)/(R_(1))-(1)/(R_(2)))`
`(1)/(f)=(1.5-1)((1)/(20)-(1)/(30))`
`(1)/(f)=0.5((30-20)/(600))`
or `(1)/(f)=(1)/(2)xx(10)/(600)=(1)/(20)` or `f=120cm`