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For a system of two separated lenses of focal lengths f1 and f2, separation for the achromatism should be: 

1. \(\dfrac{f_1-f_2}{2}\)
2. f1 + f2
3. f1 - f2
4. \(\dfrac{f_1+f_2}{2}\)

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Correct Answer - Option 4 : \(\dfrac{f_1+f_2}{2}\)


  • Achromatism: It is the phenomenon by virtue of which an object seen in white light through a lens would appear white i.e.there is no colour splitting
    • Condition: To achieve achromatism, all colours should be focused at a single point.
      • To produce an achromatic lens, a convex lens with positive chromatic aberration should be combined with a suitable concave lens of negative chromatic aberration so that total chromatic aberration is zero.
    • Application: It is a lens that is designed to limit the effects of chromatic and spherical aberration. 
      • Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus on the same plane.


  • To minimize chromatic aberration the distance between two lenses should be the average of their focal length.

So, here given focal lengths are f1​ and f2

The average of both the focal lengths is

 \( {f_{avg}} = \frac{{{f_1} + {f_2}}}{2}{\rm{ }}\)

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