what is abbe number

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In 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) in relation to the refractive index. It is represents by V_D  and with high values of V_D indicating low dispersion (low chromatic aberration). It is named after Ernst Abbe, the German physicist who defined it. The Abbe number of a material is defined as
$$\begin{gathered} V_D=\frac{n_D-1}{n_F-n_C} \\ where n_D, n_F and n_C are the refractive indices of the material at the wavelength \\ of Fraunhofer D, F and C spectral lines (589.3 nm, 486.1 nm and 656.3 nm respectively) \end{gathered}$$