# Photon in refractive medium

When light enters a refractive medium, is speed changes according to the refractive index $n$ of the medium. The microscopic explanation for this relies on calculations involving waves.

There should also be an explanation in terms of photons, no?

Why does a photon slow down when it is in a refractive medium?

A photon is a state of the EM quantum field in vacuum. This state is massless and travels at $c$.

However when electrons are around the interaction between the photon and the electron entangles the photon state with the electron states and we end up with a mixed state called a polariton. This is a quasiparticle with an effective mass greater than zero so it travels at less than $c$.

This is why photons in a dielectric medium travel at slower than the speed of light. It's because they are no longer photons but instead a mixed state of the photon with the electrons in the dielectric. However under normal circumstances the polariton definition is not a useful one because the mixing is small. It's only when the interaction is very strong, e.g. in a Bose-Einstein condensate, that it's useful to describe the propagation of the light in terms of polaritons. Under most circumstances the calssical treatment is the simplest and most accurate, and that's why it's normally used.

This is almost an exact duplicate of What is the mechanism behind the slowdown of light/photons in a transparent medium?.

However, this question is about the particle model and its relation to refraction.

The short answer is that sometimes it seems better to model light as a particle, sometimes as a wave; in this case it makes sense for most to model light as a wave. However, we must remember that the wave and particle models of light are just that; models. We find them as very handy models to describe our observations of light, and to make predictions based on those models, however ultimately the nature of light is quite an elusive concept. We seem to know a lot about, and at the same time don't know a lot about, light.

Nevertheless, a photon itself can be thought of as having its own wave signature describing its motion. In that case you come full circle to thinking about the interaction of a wave with a material again.

• It's true that the question you linked to is similar, but all the answers are in terms of waves. The wave explanations is satisfying to me, but I want to know if the effect can be described in terms of photons, not waves, My question is specific about this, so I'd argue that it should stay. – psitae Sep 25 '17 at 7:22
• Yes I agree it should stay which is why I have had a go at an answer. This is why I said it is 'almost' a duplicate. However, all 'particles' themselves have their own wave signature, so the photon is a particle not a wave in this sense, but its interaction with the material (refraction) is described by a wave function, if that makes sense. I have attacked this from more of a classical sense. John Rennie's answer provides some more illumination on this topic however that is beyond my level. – L. Maynard Sep 25 '17 at 7:25