〈 Berger | Dillon 〉

But ELI5 why different colors/frequencies have different n’s?

Depending on what level of description you're looking for it's rather nontrivial. But you you can think of it as the atoms of the glass have particular "resonant" frequencies.. Being made of charged particles, they will interact with the light in slightly different ways

That’s probably the best particle interpretation. To me it seems cleanest to think in terms of waves here though. Boundary conditions require wave to be continuous at boundary, and since wave velocity changes upon entering medium, wavelength must change?

These are all consistent, but I prefer to think about them more fundamentally

But it doesn’t really make sense to discuss a single bare photon in a material. It reduces system to a vacuum. And, diamond (~2.4) and ash (~1.1) are both just carbon but have greatly different n’s. The index of refraction depends equally on the geometry as it does atom type.