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

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.

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.