The Abbe number, VD, of a material is defined as V D = n D − 1 n F − n C , {\displaystyle V_{D}={\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 (589.3 nm, 486.1 nm and 656.3 nm respectively).
Sendo o número de Abbe, V: V = n D − 1 n F − n C , {\displaystyle V={\frac {n_{D}-1}{n_{F}-n_{C}}},} onde nD, nF e nC son os índices de refracción do material nas lonxitudes de onda correspondentes as liñas de Fraunhofer D-, F- e C- (589,2 nm, 486,1 nm e 656,3 nm respectivamente).