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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:

$V = \frac{ n_d - 1 }{ n_F - n_C }$

where nd, nF and nC are the refractive indices of the material at the wavelengths of the Fraunhofer[?] d-, F- and C- spectral lines (587.6 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. 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.

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