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07-05-2012, 14:03   #1
krd
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Light through a transparent medium - is it a phonon or a photon?

Something I saw on the physics forum.

http://www.physicsforums.com/archive.../t-511177.html

As an answer to why light is slower through a medium, than a vacuum.

From what I can make out, it says, that when light hits something like glass, the photon becomes a phonon - effectively it's absorbed by the medium - then re-emmerges at the other side as a photon.


I know what a phonon is in terms of sound. Is the physics forum correct? I would have thought a phonon, couldn't move that fast.

Actually are they completely wrong?.......Or are they right?....does it explain reflection?.......A incident photon hits a reflective surface, the surface reacts with a phonon, which then releases a photon - which sounds wrongs as well, as would you expect a slight phase shift in inferometry, and I don't remember having to account for one.
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08-05-2012, 14:14   #2
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My understanding is that it remains a photon (the same photon) all the way through. Those photons that don't make it through the transparent medium become phonons when they are absorbed, they give their energy up to the medium as heat (i.e. vibrations, i.e. phonons).

So a sheet of glass with 95% transparency will turn 5% of photons into phonons and 95% will pass through.
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08-05-2012, 22:31   #3
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My understanding is that it remains a photon (the same photon) all the way through. Those photons that don't make it through the transparent medium become phonons when they are absorbed, they give their energy up to the medium as heat (i.e. vibrations, i.e. phonons).
Well the Physics forum post is saying something else. What it appears to be saying is the light wave enters the transparent material, which is not a vacuum and then becomes a phonon (this is to explain why it travels slower than the speed of light.)


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So a sheet of glass with 95% transparency will turn 5% of photons into phonons and 95% will pass through.
That's a different phonon. That's the wave function of the light collapsing - like light passing through a coloured filter. The light outside the wavelength of the light the filter passes will collapse - and then rest will find its way into phonons, but not the same thing.

The description on the physics forum is something else. I'm not sure it's correct...It more or less says, that once the light wave enters the material, it collapses, but it's retained in a phonon, similar shape, but slower velocity, because it is not light - then emerges the other end as light.
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09-05-2012, 14:11   #4
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Surely a phonon would be restricted to the speed of sound in any given material.
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09-05-2012, 17:06   #5
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Surely a phonon would be restricted to the speed of sound in any given material.
Yeah.......Ya see, I'm not all that sure about this whole thing. They don't really give a detailed description of what the phonon transmitting the light would be.

It definitely couldn't be the same thing as sound phonons. All sound is transmitted in phonons - sound can't travel through a vacuum, because there's no medium for the phonons to move through.

Sound waves are also longitudinal. Sound does something strange when hits a solid - like the wall of a boom time "luxury" apartment. What doesn't get absorbed, once it re-emerges at the other side, will be sound wave.

Different explanations I've heard of how light passes through a transparent medium don't make sense. Like, that there is enough room in the crystal lattice to allow light to sneak its way through. But that would be empty vacuum space - why would the speed of the light be slower?......Is it some kind of gravitational effect - or are the light waves being bent by the electro magnetic field of the atoms in the crystal lattice.

Or is it a phonon. Is it quantum tunnelling.

If the phonon idea is correct, it would mean that the energy of the electrons in the transparent medium is vibrating at just a little bit lower than the frequency and amplitude of the light it's transmitting. Then again, it doesn't make complete sense, as a certain amount of the light passing through the medium does behave as if it is photons. But then again, maybe the phonon is so close to a photon, it will behave like one.


It's a mad idea. I don't know the quantum stuff well enough to know if this is possible or absolutely impossible. It's definitely not the same thing as an excited electron releasing a photon.

It raises other questions. I think it would explain the reflection of light from a mirror in terms of Newton's Third law of motion. The light hits the mirror - the material responds with a phonon - and the phonon quickly becomes a photon.

I have a really strong feeling that I'm completely wrong - but I don't know enough to say I'm completely wrong for sure.
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09-05-2012, 17:42   #6
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God, phonon's an awful word. Just doesn't sound nice at all.
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09-05-2012, 19:48   #7
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God, phonon's an awful word. Just doesn't sound nice at all.
Well, I knew these things existed. But only heard the name phonon a few months back.

They do exist - but how far the idea goes is another thing.

The thought has just crossed my mind of an electrical phonon - electron enters one end of a conductor - is converted into a charge phonon - the charge phonon progresses to the end of the conductor, and becomes an electron.

I have a strong feeling, that idea is in absolute violation of standard accepted physics.

At the same time, I can't think of anyway you could prove it's not happening. In fact, I think it is happening. In the way electricity can tunnel it's way through a conductor (like in the crimped heads of an ethernet cable - the pins never break the plastic insulator). So, it's not electrons passing the gap, but charge phonons are.
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10-05-2012, 10:03   #8
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The thought has just crossed my mind of an electrical phonon - electron enters one end of a conductor - is converted into a charge phonon - the charge phonon progresses to the end of the conductor, and becomes an electron.
A phonon is a quantization of mechanical vibration (including heat and sound), nothing to do with charged particles or electric fields.
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10-05-2012, 13:30   #9
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A phonon is a quantization of mechanical vibration (including heat and sound), nothing to do with charged particles or electric fields.
Why wouldn't the mechanical vibration have an effect on the electric fields?

Since charge can be induced through mechanical action, what's to stop charge being carried by phonons?
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10-05-2012, 13:53   #10
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All the possible paths and interactions through the medium are computed, including the path where the photon doesn't interact with the medium at all, as they all contribute to the probability amplitude of the photon reaching some detector placed behind the material.
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10-05-2012, 15:00   #11
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All the possible paths and interactions through the medium are computed, including the path where the photon doesn't interact with the medium at all, as they all contribute to the probability amplitude of the photon reaching some detector placed behind the material.
But you could say the same thing, if there was no transparent medium.

The light that passes through the medium is slower than the speed of light, by what mechanism is its path becoming longer. If most of the photon is passing through empty space unpeturbed, why should it be slower.

The idea on the physics forum, is the light wave enters the transparent material - but it's no longer a photon, it becomes a phonon moving through the electrical field of the transparent material, which makes it go slower than light. It re-emerges at the other end as a photon. It hits the detector as a photon. .........Although something completely different. When sound enters a thin wall, it's characteristics change, when it re-emerges it becomes sound again.

I'm not sure where the idea comes from. I've been trying to look up stuff on it, and it may have been one of Einsteins ideas. I'm not sure there is a way of proving it does or doesn't happen. Unless there's a very solid reason why it cannot happen.

The idea would appear to be, that light through a transparent medium is actually a vibration in the medium, not photons passing through empty space.
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22-05-2012, 10:18   #12
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I think although I have some bits wrong, I'm right.

Here's Phil Moriarty on 60 symbols explaining light transmission through an transparent medium.




Phil says, the photons that pass through the medium, do not have enough energy to push the electrons over the energy gap and get absorbed, but the way he describes it (waving his hands in the air) the electrons seem to oscillate beneath the energy gap. Which if I'm understanding this correctly means that the light passing through the medium is not the self propagating photon that travels through a vacuum, but is a wave propagated through a medium - the medium being electrons. Which would make it a phonon and not a photon.

I really can't remember this stuff properly since it's so long since I studied it. But, I thought - for whatever reason - the electrons could only jump from one gap to another, and there wasn't an intermediary stage. Phil's description would indicate that the electrons move like water molecules (kind of)
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