Question

The dispersive powers of crown and flint glasses are `0.02` and `0.04` respectively. In an achromatic combination of lenses the focal length of flint glass lens is `40 cm`. The focal length of crown glass lens will be
A. `-20 cm`
B. `+20 cm`
C. `-10 cm`
D. `+10 cm`

Answer 1

Correct Answer - A
`(omega_(1))/(f_(1)) +(omega_(2))/(f_(2)) = 0 rArr (0.02)/(f_(1)) +(0.04)/(40) = 0`
`f_(1) =- 20cm`