The ratio of refractivity to dispersion in an optical medium. Specifically: (nd-1)/(nF-nC), where n is the index of refraction for the Fraunhofer d, F and C lines, respectively.
denotes the degree of refraction of light of different wavelengths to different extents, given by a transparent material, such as glass. The lower the Abbe number, the greater the dispersion of colors.
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). It is named for Ernst Abbe (1840–1905), the German physicist who defined it.