# 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

*n*_{D},

*n*_{F} and

*n*_{C} 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.