Question

Two plano-convex lenses of same material and of focal lengths $f$ and $2f$ are coaxially separated by a suitable distace to minimize chromatic aberration. The equivalent focal length of the combination is

• A. $f$
• B. $3f/2$
• C. $3f/4$
• D. $4f/3$

Answer 1

The correct option is D $4f/3$

$d=\frac{f_2+f_1}{2}=\frac{2f+f}{2}=\frac{3f}{2}$

$f_{eq}=\frac{f\times 2f}{\frac{-3f}{2}+2f+f}=\frac{2f^2}{\frac{3f}{2}}=\frac{4f}{3}$