Question

A concavo-convex lens has refractive index `1.5` and the radii of curvature of its surfaces are `10 cm and 20 cm.` The concave surface is upwords and is filled with oil of refractive index `1.6.` The focal length of the combination will be
A. `18.18 cm`
B. `15 cm`
C. `22 cm`
D. `28.57 cm`

Answer 1

Correct Answer - D
`1/F=1/(f_1)+1/(f_2)`

`=(1.6-1)(1/oo-1/(-10))(1.5-1)(1/(-10)-1/(-20))`
Solving, we get, `F=28.57 cm`