The elimination of chromatic aberration by combining a positive lens of one material with a negative lens of another material is called what?

• A. Achromatic doublet
• B. Apochromatic triplet
• C. Monochromatic lens
• D. Plano-convex configuration

Answer: (A)

Chromatic aberration happens because different colors bend by different amounts in glass, so colors focus at different distances. By pairing a positive lens made of one material with a negative lens made of another material, you can balance how each color bends so that two wavelengths come to nearly the same focus. This configuration is called an achromatic doublet. The typical pairing uses a low-dispersion positive crown glass and a higher-dispersion negative flint glass, chosen so the focal shift between colors (often red and violet) cancels out, greatly reducing longitudinal chromatic aberration and producing sharper, more color-accurate images.

An apochromatic triplet would go further by correcting more wavelengths with three elements, but the described two-material, two-lens setup is the classic achromatic doublet. A monochromatic lens or a plano-convex configuration alone doesn’t describe this correction method, since shape or focusing of a single color doesn’t inherently fix color-dependent focusing errors.