Red light has longer wavelength than say blue light and so bends the least as it travels much faster in the same medium. My question is since energy is not stated in the above explanation, is QM required for prism to work?
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$\begingroup$ You also get dispersion in a flat piece of glass. You don't notice it because the exiting light comes back together to form WhiteLight $\endgroup$– Bill AlseptCommented Oct 16, 2017 at 17:02
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You are right! Classical electromagnetic theory is sufficient to describe how light refracts at an interface (Snell's law). Then, if you stipulate that refractive index is frequency-dependent, you get the light-dispersing behavior of a prism. Of course, if you want to know why there is dispersion in glass, you have to go a little deeper, possibly explaining it with a classical electron oscillator model of the polarizability. But depending on how accurate you want to be, you might end up going down a road that ends with QM.
