Question

Two lenses have focal lengths $f_{1} \& f_{2}$ and their dispersive powers are $\omega_{1} \& \omega_{2}$ respectively. They will together form an achromatic combination if:

• A. $\omega_{1} f_{1}=w_{2} f_{2}$
• B. $\omega_{1} f _{2}+\omega_{2} f _{1}=0$
• C. $\omega_{1}+f_{1}=\omega_{1} f_{2}$
• D. $\omega_{1}-f_{1}=\omega_{2}-f_{2}$

Answer 1

Answer: (B)

For achromatic combination
$\frac{\omega_{1}}{f_{1}}+\frac{\omega_{2}}{f_{2}}=0$
$\therefore \omega_{1} f_{2}+\omega_{2} f_{1}=0$