15.
What condition must be satisfied by two thin lenses kept in contact so that they form an achromatic doublet, i.e. a combination free from chromatic aberration ?
Condition for achromatic doublet of two lens:
![straight omega over straight f space plus space fraction numerator omega over denominator f apostrophe end fraction space equals space 0 space](/application/zrc/images/qvar/IPHEN12112676.png)
are the dispersive power of two lens,
f + f' are the respective focal length.
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