Q. 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) $+ 10\ cm$
Explanation
(a) By using $\frac{\omega_{1}}{f_{1}} + \frac{\omega_{2}}{f_{2}} = 0 \Rightarrow \frac{0.02}{f_{1}} + \frac{0.04}{40} = 0$
$f_{1} = - 20\ cm$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.