Q. Two lenses have focal lengths $f_{1}$ and $f_{2}$ and their dispersive powers are $\omega_{1}$ and $\omega_{2}$ respectively. They will together from an achromatic combination if
A) $\omega_{1}f_{1} = \omega_{2}f_{2}$
B) $\omega_{1}f_{2} + \omega_{2}f_{1} = 0$
C) $\omega_{1} + f_{1} = \omega_{2} + f_{2}$
D) $\omega_{1} - f_{1} = \omega_{2} - f_{2}$
Explanation
(b) For achromatic combination, $\frac{\omega_{1}}{f_{1}} + \frac{\omega_{2}}{f_{2}} = 0$
$\Rightarrow \omega_{1}f_{2} + \omega_{2}f_{1} = 0$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.