Q. The dispersive powers of glasses of lenses used in an achromatic pair are in the ratio $5\ :3$. If the focal length of the concave lens is $15\ cm$, then the nature and focal length of the other lens would be
A) Convex, $9\ cm$
B) Concave, $9\ cm$
C) Convex, $25\ cm$
D) Concave, $25\ cm$
Explanation
(a) $\frac{\omega_{1}}{\omega_{2}} = - \frac{f_{1}}{f_{2}} \Rightarrow \frac{5}{3} = \frac{- ( - 15)}{f_{2}} \Rightarrow f_{2} = 9\ cm$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.