Question

A combination of two thin lenses of the same material with focal lengths f1 and f2, arranged on a common axis minimizes chromatic aberration, if the distance between them is:

• A. $ \frac{\left( {{f}_{1}}+{{f}_{2}} \right)}{4} $
• B. $ \frac{\left( {{f}_{1}}+{{f}_{2}} \right)}{2} $
• C. $ \left( {{f}_{1}}+{{f}_{2}} \right) $
• D. $ 2\left( {{f}_{1}}+{{f}_{2}} \right) $

Answer 1

Answer: (B)

Condition for achromatism for two thin convex lenses of same material and mean focal lengths $ {{f}_{1}} $ and $ {{f}_{2}} $ placed at a distance d apart form an achromatic combination provided, $ d=\frac{{{f}_{1}}+{{f}_{2}}}{2} $ This combination is free from lateral chromatic aberration for all colours.