The ISO 9802 Raw Optical Glass–Vocabulary and ISO 12123 Specification of Raw Optical Glass define refractive index as follows: “Ratio of the velocity of the electromagnetic waves at a specific wavelength in a vacuum to the velocity of the waves in the medium.”
Dispersion denotes the effect that refractive index changes with wavelength. For optical glasses, refractive index changes more strongly in the blue–violet range than in the red range of the visible (VIS) spectrum (see Fig. 3.2). The dispersion curve is not linear and is also not a simple function of the wavelength. In order to easily characterize glass types with respect to their dispersion behavior, one uses different simplifying quantities in practice.
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