Hmm, how is the wave speed of a wave different from its phase velocity? I'm assuming the latter is the speed at which a wave front 'moves'?

Nevermind, they are usually the same thing.

did you die?

I haven't seen you in a week.

@0celo7 well.. I was 'busy' so I couldn't go on very often.

I should be back regularly again. also, @danS can you briefly explain the process in this answer physics.stackexchange.com/questions/105573/… ? What is the process called, specifically?

(The absorption and emission of the photons, that is) if it has a name

Oh right how's your eye @0celo7

@0celo7 What is here?

@acuriousmind hey can you answer the question I posed to Dan?

(Or anyone that feels like it)

What you linked there is not an answer, but a question. What process exactly are you talking about?

The answer given by Martin introduces a process where atoms in the medium absorb, then re-emit light that comes in contact with it. I want to know what this process is.

It's...the process of absorption and re-emission

I don't think it has a particular name

Except for "light travelling through a medium", that is :P

@acuriousmind is there an underlying process involved? It seems to suggest this because it takes 'time' for this to happen.

Well, absorption and emission of light by atoms is not instantaneous, the atom spends some short time in an excited state in between.

Then again, a comment says in classical EM theory, there is no time delay.

@acuriousmind Is it the same amount of time each time?

(spent in the excited state)

Roughly, but it's going to fluctuate a bit. The "constant" speed only comes about on average in this picture.

I wonder what factors influence the time spent in this excited state, then..

Anyway, I'm not entirely convinced this picture of absorption of re-emission is really the best way you should picture light in a medium on the level of photons. Another way would be to consider the medium giving the photon an effective mass, thereby slowing it down.

@Obliv Nothing, the decay of excited states is random, i.e. we can only give half-lives.

Oh, that would be decay?

Didn't realize that :o

@acuriousmind why would giving the light mass slow it down?

Oh nvm..

lol

@Obliv Because massive objects can't travel at the speed of light?

I don't really know whether the effective mass idea works for a photon, but it's what other excitations in solids (e.g. phonons, holes) acquire.

Well, that works for any mechanical wave, right? I don't think it should be restricted to waves in solids @acuriousmind

I don't know what you mean by a "mechanical wave"

a wave that travels in a medium (be it a liquid, gas, solid)

Well, I said "solid" because that's what one typically looks at in condensed matter at first, and I've never seen a QM-like treatment of excitations in liquids/gases, but that's really just because it's not my area