This is, I think, just one of those spots where "all models are wrong, some models are useful" is true. From the wikipedia page on Refractive Index (https://en.

This is, I think, just one of those spots where "all models are wrong, some models are useful" is true. From the wikipedia page on Refractive Index (https://en.wikipedia.org/wiki/Refractive_index https://en.wikipedia.org/wiki/Refractive_index): > The refractive index may vary with wavelength. This causes white light to split into constituent colors when refracted. This is called dispersion. This effect can be observed in prisms and rainbows, and as chromatic aberration in lenses. --- snip --- > For most materials the refractive index changes with wavelength by several percent across the visible spectrum. Nevertheless, refractive indices for materials are commonly reported using a single value for n, typically measured at 633 nm. So we were all lied to in our introductory optics classes. n isn't a constant for a given material but rather n(lambda) but weakly.