Question

Two lenses of focal lengths $+10$ cm and $-15$cm when put in contact behave like a convex lens. They will have zero longitudinal chromatic aberration if their dispersive powers are in the ratio

• A. $+3/2$
• B. $+2/3$
• C. $-3/2$
• D. $-1/2$

Answer 1

The correct option is B $+2/3$

For achromatic aberration:
$\frac{w_1}{f_1}+\frac{w_2}{f_2}=0$
$\frac{w_1}{w_2}=\frac{-f_1}{f_2}$
$=\frac{-10}{(-15)}$
$=\frac{+2}{3}$