Question

Two lenses in contact form an achromatic lens. Their focal lengths are in the ratio 2 : 3. Their dispersive powers must be in the ratio of

• A. 1 : 3
• B. 2 : 3
• C. 3 : 2
• D. 3 : 1

Answer 1

Answer: (B)

2 : 3

Condition for achromatism is $\frac{{\omega }_1}{f_1}+\frac{{\omega }_2}{f_2}=0$

$\therefore \frac{f_1}{f_2}=-\frac{{\omega }_1}{{\omega }_2}=\frac{2}{3}$ (leaving sign)