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Abbe number

In physics and optics, the Abbe number, also known as the V-number or constringence of a transparent material is a measure of the material's dispersion (variation of refractive index with wavelength). Named for Ernst Abbe (1840-1905), German physicist.

The Abbe number V of a material is defined as:

where nD, nF and nC are the refractive indices of the material at the wavelengths of the Fraunhofer D-, F- and C- spectral lines (589.2 nm, 486.1 nm and 656.3 nm respectively). Low dispersion materials have high values of V.

Abbe numbers are used to classify glasses, for example flint glasses have V<50 and crown glasses V >50. Typical values of V range from around 20 for very dense flint glasses, up to 60 for very light crown glass. Abbe numbers are only a useful measure of dispersion for visible light, and for other wavelengths, or for higher precision work, the group velocity dispersion is used.