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) \[2\left( {{f}_{1}}+{{f}_{2}} \right)\]
Correct Answer: B
Solution :
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.
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