Tutorial

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 certain new kinds of nanophotonics crystals), n is larger than or equal to "1".

n depends on the wavelength and decreases when the wavelength increases. Consequently, when defining the refraction index by a reference of a glass catalog instead of a numerical value, it is necessary to specify the wavelength. The refraction index used for the calculation is then the one corresponding to the selected wavelength.

When using a glass catalog reference, the refraction index is calculated according the Sellmeier formula:

where B_i and C_i are coefficients relevant to the considered glass.

By default, the wavelength is 0.55 μm. It is possible to specify another wavelength in the general parameters/wavelengths page.

The general parameters/wavelengths page enables also to define a small and a large wavelength (respectively lam_{_min} and lam_{_max}) that are considered in the calculation pages related to the "chromatic aberration" when refraction indexes are specified by a glass reference instead of numeric values. The small and large wavelengths define then the spectral range in which the chromatism is computed.


Constringence

The consringence or Abbe number characterizes the dispersion of a material in a certain spectral range by the formula :

v_d = (n_d - 1)/ (n_F- n_C).

n_F and n_C are the refraction indexes of the material respectively at 486,13 nm and 656,27 nm, n_d being the refraction index at the intermediate wavelength of 587,56 nm.


Another definition of the Abbe number is :

v_e = (n_e - 1)/ (n_F'- n_C').

n_F' and n_C' are the refraction indexes of the material respectively at 480,0 nm and 643,8 nm, n_e 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.