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.

Related Topics:
Physics - Optics - Dispersion - Refractive index - Ernst Abbe - 1840 - 1905

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The Abbe number V of a material is defined as:

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:V = rac{ n_D - 1 }{ n_F - n_C }

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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.

Related Topics:
Refractive indices - Fraunhofer - Spectral line - Nm

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Abbe numbers are used to classify glasses. For example, flint glasses have V50. Typical values of V range from around 20 for very dense flint glasses, up to 65 for very light crown glass, and up to 85 for fluor-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.

Related Topics:
Glass - Flint glass - Crown glass - Fluor - Group velocity dispersion

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See also:

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• Abbe prism
• Abbe refractometer