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Refraction index

The refraction index n of a medium in which light can propagate is the ratio between the light speed in vacuum ( or air) and the light speed in the considered medium. Because light speed is higher in vacuum ( or air ) than in any other medium ( except perhaps certain new kinds of media like some nanophotonics crystals ), n is larger than 1.
n depends on the wavelength and decreases when the wavelength increases. Consequently, when selecting a material reference of the glass catalog ( instead of enterring the refraction index) in a calculation page, it's necessary to specify the wavelength. The refraction index used for the calculation will then correspond to the selected wavelength. By default, the wavelength is 0.55 µm. It is possible to specify another wavelength in the def. wavelength page.

When using two wavelengths, as for chromatism calculation, def. wavelength page allows to enter a small and a large wavelength ( respectively lam_min and lam_max ). If material references are used, the low and high refraction indexes will correspond respectively to the small and large wavelengths.

Constringence.

The consringence or Abbe number ( vd or ve in the calculation pages ) characterizes the dispersion of a material in a certain spectral range by the formula :

vd = ( nd - 1 )/ ( nF- nC )

where nF and nC are the refraction indexes of the material respectively at 486,13 nm and 656,27 nm, nd being the refraction index at the intermediate wavelength of 587,56 nm.

Another definition of the Abbe number is :

ve = ( ne - 1 )/ ( nF'- nC' )

where nF' and nC' are the refraction indexes of the material respectively at 480,0 nm and 643,8 nm, ne being the refraction index at the intermediate wavelength of 546.07 nm.

The lower the constringence, the more dispersive the material. Generally, materials are classified in three categories : Flint glasses ( high dispersion ) with Abbe number smaller than 39, intermediate glasses with Abbe number between 39 and 45, and Crown glasses ( low dispersion ) with Abbe number larger than 45.

References

"Optique Fondements et applications" - 2004 - author : José-Philippe Perez.

"Optique géométrique Imagerie et instruments" - 2007 - author : Bernard Balland.

"Optique géométrique paraxiale" - Institut d'optique théorique et appliquée - 1985 - author : Michel Cagnet.

"Formation des images Aberrations" - Institut d'optique théorique et appliquée - 1985 - author : Michel Cagnet.



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