Achromatic lens

An achromatic lens is a compound lens designed to bring two wavelengths of light to nearly the same focus, reducing chromatic aberration.

Chromatic aberration: An optical defect in which different wavelengths form images at different positions or magnifications. The color fringing it produces is the primary defect an achromatic lens is designed to reduce.
Refraction: The change in direction of a wave as it passes between materials with different wave speeds. An achromatic lens corrects color focus by balancing wavelength-dependent refraction.
Apochromatic lens: A lens corrected to bring three wavelengths to a common focus and control secondary spectrum. It corrects more colors than an achromat, typically with greater optical complexity.
Refracting telescope: A telescope that forms images primarily with lenses. Its objective often uses an achromatic doublet to limit colored halos around astronomical objects.
Dispersion: The wavelength dependence of a material’s refractive index, causing different colors to bend by different amounts. Different dispersions in paired glasses let the lens counterbalance color-dependent focusing.
Refractive index: A measure of how much slower light travels in a material than in vacuum. Its wavelength dependence determines how strongly each glass element bends each color.
Monochromatic aberration: An image defect that occurs even when light is treated as a single wavelength. Achromatic correction targets color-dependent error, not these wavelength-independent aberrations.
Achromatic objective: A microscope objective corrected for chromatic focus differences at selected wavelengths. It applies achromatic correction to the image-forming front end of a microscope.
Crown glass: A family of optical glasses with relatively low refractive index and low dispersion. It commonly supplies the low-dispersion element in a classical achromatic doublet.
Focal length: The distance from an optical system’s principal plane to its focal point for parallel incoming rays. Achromatic correction means matching focal positions for selected wavelengths.

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