For one given transparent medium, how does the index of refraction vary across the visible spectrum?

1. For a given medium, the index of refraction increases as the wavelength decreases. 2. Violet light, at about 410 nm, has the shortest visible wavelength and so meets the largest index. 3. In crown glass, for instance, the index runs from 1.512 for red at 660 nm up to 1.530 for violet at 410 nm. 4. Because the index climbs steadily with falling wavelength, the colours emerge in the fixed red to violet order. 5. If the index were the same at every wavelength there would be no dispersion and no rainbow. _Source: OpenStax College Physics (CC BY 4.0), Ch 25 "Geometric Optics", section 25.5 Dispersion: The Rainbow and Prisms_