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.