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Obliv

23:05

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.

0celo7

did you die?

I haven't seen you in a week.

Obliv

@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

ACuriousMind

@0celo7 What is here?

Obliv

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

(Or anyone that feels like it)

ACuriousMind

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

Obliv

23:22

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.

ACuriousMind

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

Obliv

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

ACuriousMind

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

Obliv

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)

ACuriousMind

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

Obliv

23:26

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

ACuriousMind

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.

Obliv

Oh, that would be decay?

Didn't realize that :o

heather

@DanielSank, okay, thank you! My dad said I could be there at 2:00-2:30 in the afternoon on Tuesday; does that work for you?

Obliv

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

Oh nvm..

lol

ACuriousMind

@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.

Obliv

23:30

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

ACuriousMind

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

Obliv

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

ACuriousMind

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

Physics Meta

Q: Please re-examine this question

I'd like to strongly suggest that the powers that be on this site carefully re-examine the following question and the reasons it was put on hold or regarded as not a good enough question: Varying angular frequency in SHM OP's question about the potential energy being $|x|^3$ but it then describ...