Which optical combination is used to reduce chromatic aberration by mixing materials with different dispersion properties?

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

Answer: (A)

Chromatic aberration comes from the fact that a lens’ refractive index changes with wavelength, so different colors come to focus at different distances. To counter this, you mix materials with different dispersion so their focal-length changes cancel each other out for at least two wavelengths. An achromatic doublet does exactly that: a positive lens of crown glass (low dispersion) and a negative lens of flint glass (high dispersion) are chosen so their chromatic focal shifts balance. The result is a lens system that brings two wavelengths to nearly the same focus, greatly reducing longitudinal chromatic aberration across the spectrum.

The other options don’t apply this mixing approach. A monochromatic doublet is designed for a single wavelength and won’t correct color-induced focus shifts. A plano-convex lens is a single element with no combination to cancel dispersion. An apochromatic triplet uses three elements and goes further to correct chromatic aberration across three wavelengths, but the standard, simplest color-correction with two differing glasses is the achromatic doublet.