Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2019/iii/paper-338/3/b/i/solution

For thin lenses in contact, optical powers add. At the two reference wavelengths, the lensmaker's equation gives total power
An achromatic lens has . Subtracting and solving for the curvature-factor ratio yields
Opposite curvature-weighted powers compensate the two materials' optical dispersion.

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