`V_D = ( n_D -1)/( n_F - n_C )`

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The Abbe number, also known as the V-number of constringence of a transparent material,  is a measure of the material's dispersion (variation of refractive index versus wavelength), with high values of `V` indicating low dispersion. The following formula is used: `V_D=(n_D-1)/(n_F-n_C)`, where:

• `n_D` = refractive index of material at the wavelength of Fraunhofer D- spectral line
• `n_F` = refractive index of material at the wavelength of Fraunhofer F- spectral line
• `n_C` = refractive index of material at the wavelength of Fraunhofer C- spectral line

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Abbe numbers are used to classify glass and other optical materials in terms of their chromaticity. For example, the higher dispersion flint glasses have `V`<55 whereas the lower dispersion crown glasses have larger Abbe numbers. Values of `V` range from below 25 for very dense flint glasses, around 34 for polycarbonate plastics, up to 65 for common crown glasses, and 75 to 85 for some fluorite and phosphate crown glasses. 

Abbe numbers are used in the design of achromatic lenses, as their reciprocal is proportional to dispersion (slope of refractive index versus wavelength) in the wavelength region where the human eyes is most sensitive. For different wavelength regions, or for higher precision in characterizing a system's chromaticity (such as in the design of apochromats), the full dispersion relation (refractive index as a function of wavelength) is used. 

References

Wikipedia (https://en.wikipedia.org/wiki/Abbe_number)