Previous studies have shown that the Lorentz–Lorenz relationship, or molar refractivity/specific refractivity effective medium approximation, enables a reasonable prediction of the refractive index of amorphous water ice, given the refractive index of crystalline water ice. In the current study, we show that the Bruggeman effective medium approximation provides an even closer match to measurements of the refractive index of several amorphous materials, given the refractive index of their crystalline phase. We show that the Bruggeman effective medium approximation provides a good match to measurements of the refractive index of amorphous ice as well. Thus, assuming that the volume fraction of the scattering centers is a constant for a given amorphous material (with respect to a given range of wavelengths) seems to be a more robust assumption than assuming that the molar mass and molar refractivity or specific refractivity are preserved in going from the crystalline state to the amorphous state of the same material. Our results have implications for astrophysics applications, as well as for the optics of non-crystalline materials in general.
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