Why does light slow down in a medium?

nasu

A major difference between the sound phonon and the light phonon is the light phonon is travelling at relativistic speeds. It's travelling near the speed of light, not near the speed of sound.
Are talking about phonons in optical branches when you say "light phonon"?
If you do, they may have group velocities even lower than these of the phonons in acoustic branches.
If you mean something else, I would appreciate some reference.

Imagine if I had a crystal - all the atoms being held in place by their electric fields - imagine these fields are made of some kind of elastic and flexible material. If I bob one atom against another, they'll giggle up and down. There will be a little oscillation. That oscillation will not be restricted to those two atoms, it will spread through electric fields of the nearby atoms, and they'll spread it to their neighbours. If it reaches the edge of the crystal, and escapes, it becomes light.
You mean that every piece of solid material glows due to this light?

Just to say something else - if I pass a sound wave of 60Hz through a wall - the other side of the wall must flutter at 60Hz for the sound to pass into the other room - it has to push and pull the air. I'm not sure, but I wonder would you see a 60Hz light too (I don't mean see - I mean I wonder if it's there)
What would be the meaning of "60 Hz light"? You mean an electromagnetic wave with 60 Hz frequency? Or light (visible) modulated with a 60 Hz frequency?

PS. Is it possible that when you say that photons "become" phonons you may be referring to "polaritons" in a dielectric material (describing the coupling between optical phonons and photons)?

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harrylin

[..] The sound wave always needs to a medium to propagate through - it's always a phonon. Though in a gas it travels longitudinally and in a solid transverse.
A major difference between the sound phonon and the light phonon is the light phonon is travelling at relativistic speeds. It's travelling near the speed of light, not near the speed of sound. [..]
What you say sounds neat but also sounds as if it's your own theory - and at first sight it flatly contradicts light scattering theory. As explained in the other thread, the effective speed of light in a medium is considered to be the result of light propagating at the vacuum speed through the medium, whereby the secondary electromagnetic waves that are created by mainly the oscillating electrons cause an effective delay of the resulting, compound light wave. In contrast, the literature describes photons - even those called "optical phonons" - as mechanical vibrations of the atoms. Electromagnetic waves and mechanical waves can be coupled, but are by far not the same thing.

krd

What you say sounds neat but also sounds as if it's your own theory - and at first sight it flatly contradicts light scattering theory. As explained in the other thread, the effective speed of light in a medium is considered to be the result of light propagating at the vacuum speed through the medium, whereby the secondary electromagnetic waves that are created by mainly the oscillating electrons cause an effective delay of the resulting, compound light wave. In contrast, the literature describes photons - even those called "optical phonons" - as mechanical vibrations of the atoms. Electromagnetic waves and mechanical waves can be coupled, but are by far not the same thing.
The medium is not a vacuum. If you think of the electrons are tiny points spinning around the atom, and most of their path is empty space most of the time - you might think it's a vacuum. But it's not it's - in a crystal lattice between the atoms is an electric field. That electric field is the medium.

In contrast, the literature describes photons - even those called "optical phonons" - as mechanical vibrations of the atoms.
And what is the mechanism of those mechanical vibrations of the atoms?

Electromagnetic waves and mechanical waves can be coupled, but are by far not the same thing.
If they're not the same thing, what are they?

If you take a piece of wire - and repeatedly bend it - warm it up through mechanical action. Then put it under a camera that can see the infrared spectrum - you'll see more light coming from it than before you warmed it up through bending it.

Ghassan99

Because the speed of the light depends on the Optical density ..Vacuum has no Optical density while other mediums as water and air have .

krd

Are talking about phonons in optical branches when you say "light phonon"?
If you do, they may have group velocities even lower than these of the phonons in acoustic branches.
If you mean something else, I would appreciate some reference.
Somewhere in this forum there is post with a much better description than I have. Unfortunately - I don't have the link.

You mean that every piece of solid material glows due to this light?
Yes. Every piece of solid material glows due to this light - it's black body radiation. I'm not saying it's the only means of light production - but for black body radiation it is.

What would be the meaning of "60 Hz light"? You mean an electromagnetic wave with 60 Hz frequency? Or light (visible) modulated with a 60 Hz frequency?
I was just thinking - if I vibrated a solid material at 60Hz, would I get a 60Hz light emission from the surface of the material As well as the push pull on the air. Because I would be vibrating the electric field of the material at 60Hz. Unfortunately, I'm nowhere near a lab or a well equip lab, where I could give it a go. Maybe if I went up into KHz I might be able to detect a radio wave.

PS. Is it possible that when you say that photons "become" phonons you may be referring to "polaritons" in a dielectric material (describing the coupling between optical phonons and photons)?
Ultimately, it's the same idea. You could describe it as the coupling of phonons with phonons. And the dielectric material gives you a polarised phonon. And similarly, a transparent medium like water, doesn't give you polarised phonons because it's not a dielectric material.

nasu

Somewhere in this forum there is post with a much better description than I have. Unfortunately - I don't have the link.
Do you need a link to explain what you mean by "light phonon"?

Yes. Every piece of solid material glows due to this light - it's black body radiation. I'm not saying it's the only means of light production - but for black body radiation it is.
OK, so when you say "light" you mean any electromagnetic radiation.
And your theory is that the black body radiation is just phonons that reach the boundary of the solid and get converted to outgoing photons?

Ultimately, it's the same idea. You could describe it as the coupling of phonons with phonons. And the dielectric material gives you a polarised phonon. And similarly, a transparent medium like water, doesn't give you polarised phonons because it's not a dielectric material.
You mean coupling of phonons with photons, right?

Water is not a dielectric? What do you consider it to be? A conductor? Is glass (transparent medium) a dielectric? Are there "polarized phonons" in glass? How about in (transparent) sodium chloride crystals?

harrylin

[..] And what is the mechanism of those mechanical vibrations of the atoms? [..]
Yes the mechanical vibrations are coupled to electromagnetic waves as those mechanical vibrations are based on a kind of electromagnetic springs. The point here was that the speed with which mechanical vibrations propagate depends on such things as the atom mass while the speed with which electromagnetic radiation propagates depends on vacuum permittivity and permeability - and as already explained in this thread and elsewhere, without interference from secondary electromagnetic (and not mechanical) waves, that speed would be even c inside glass.
- http://en.wikipedia.org/wiki/Vacuum_permittivity

It looks essential to me that we clearly distinguish between certain mechanical vibrations and the secondary electromagnetic waves that result from (other) mechanical vibrations. If you think otherwise, please try to explain -preferably based on reliable references- why light propagates with an effective speed of several 100'000 km/s in glass, and not with several km/s.

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wilbye

So what happens if you initiate a beam of light and split it into two. Pass one split through a piece of glass and let the other pass unhindered. Do the two beams arrive at a single target at exactly the same time or does the one that passed through the glass arrive after the unhindered one?

Whovian

So what happens if you initiate a beam of light and split it into two. Pass one split through a piece of glass and let the other pass unhindered. Do the two beams arrive at a single target at exactly the same time or does the one that passed through the glass arrive after the unhindered one?
Depends on the setup.

harrylin

So what happens if you initiate a beam of light and split it into two. Pass one split through a piece of glass and let the other pass unhindered. Do the two beams arrive at a single target at exactly the same time or does the one that passed through the glass arrive after the unhindered one?
Hi welcome to physicsforums.
The classical explanation is that (mainly) the electrons in the glass atoms interact with the light wave so that they emit secondary waves which are slightly behind in phase and which interfere with the primary wave and each other. As a result the piece of glass delays the light beam; at equal distances that beam will arrive later. You can find more detailed explanations in earlier posts.

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krd

Do you need a link to explain what you mean by "light phonon"?
No. I just think their explanation was more sound looking than mine. And some of the maths/science for the electromagnetic phonon wave is stuff I never studied.

OK, so when you say "light" you mean any electromagnetic radiation.
Yes.....what other kind of light is there.

And your theory is that the black body radiation is just phonons that reach the boundary of the solid and get converted to outgoing photons?
Yes....100%......And if you can tell me it's by some other mechanism, let's hear it.

You mean coupling of phonons with photons, right?
Tomatoes, tomaytoes......if they're going to couple their resultant waves are going to be a coupling of the same thing.

Water is not a dielectric? What do you consider it to be? A conductor?
Okay. Plain tap water is not a dielectric polariser. And if it won't conduct, just throw in a little salt.

Is glass (transparent medium) a dielectric? Are there "polarized phonons" in glass? How about in (transparent) sodium chloride crystals?
All transparent solids are crystals, some are natural polarisers and some are not. And all solids to a certain extent are transparent.

If I didn't know better, I'd say you're playing some game at trying to catch me out.

krd

It looks essential to me that we clearly distinguish between certain mechanical vibrations and the secondary electromagnetic waves that result from (other) mechanical vibrations.

If you think otherwise, please try to explain -preferably based on reliable references- why light propagates with an effective speed of several 100'000 km/s in glass, and not with several km/s.
The electromagnetic wave propagates through the medium at near the speed of light - because it enters the material at the speed of light. The sound wave propagates through the material at near the speed of sound, because it enters at near the speed of sound.

Mechanical vibrations will always result in heating. And will always result in an increase of the black body radiation of the material.

harrylin

The electromagnetic wave propagates through the medium at near the speed of light - because it enters the material at the speed of light. The sound wave propagates through the material at near the speed of sound, because it enters at near the speed of sound. [..]
Sorry but no: that's not in line with wave theory. Honestly, it doesn't make any sense. And it surely won't allow you to correctly predict either the speed of light or the speed of sound in glass!

harrylin

According to these NIST researchers, phonons and dielectric constant are deeply connected.

If you don't want to wade through the work read just the abstract and conclusions. You'll get the picture.

http://www.ctcms.nist.gov/~cockayne/papers/cockayne/catio3.pdf
Interesting! I think that this is an exceptional case but that's also useful for the general explanations:

"The dielectric response is dominated by low frequency (ν ∼ 90 cm−1 ) polar optical modes in which cation motion opposes oxygen motion"

If I understand it correctly, in that crystal the secondary light waves result mainly from the interaction of ions with the light. As it's also the interaction of ions with each other that govern acoustic waves, those two properties will be directly related - although still not the same.

nasu

If I didn't know better, I'd say you're playing some game at trying to catch me out.
I was actually trying to figure out if you are making things out or just presenting some information in a distorted way and/or mixing up the meaning of some usual terms.

Yes.....what other kind of light is there.
Well, whereas light is electromagnetic wave, not all electromagnetic waves are visible light.
Some people may use "light" to mean EM radiation in general.
I asked you if by "light" you mean visible or EM in general. Your answer show that I did nor make myself clear enough.

Tomatoes, tomaytoes......if they're going to couple their resultant waves are going to be a coupling of the same thing.
No and no. Even though "photons" and "phonons" have only one letter different, they mean very distinct things. The fact that they may couple does not automatically imply that they are the same thing of that will be a coupling of the "same thing".
Along the same line of argument, electrons and protons are the same thing if they are going to couple in a hydrogen atom (to keep the example in the field of physics).

Okay. Plain tap water is not a dielectric polariser. And if it won't conduct, just throw in a little salt.
All transparent solids are crystals, some are natural polarisers and some are not. And all solids to a certain extent are transparent.
The term "polarizer" is usually used to designate a device or filter that passes light with a specific polarization state. Some crystals (example calcite) are natural polarizers. But this is quite unrelated to the question, which was about dielectrics. Maybe you mean something else.

Pure water is considered usually a dielectric material (see dielectric constant of water).

What would be the point of "All transparent solids are crystals"? Do you mean that some of the nontransparent ones are not? Or that only the transparent ones are? I am not playing games, it's simply unclear to me what point are trying to make.

Rajashik

As far as i can comprehend it results from the interaction of electric and magnetic fields of light with molecules of the medium. Inevitably the propagation spped would slow down.
Found a good rticle on wiki:-
http://en.wikipedia.org/wiki/Slow_light

TonyClifton

Richard P. Feynman explains these topics very well, I advice you all to view his lectures on QED (available on youtube). To understand this question you have to know a lititle bit about quantum mechanics.

Light is an electro magnetic wave but when 1 photon is followed it's a particle, this is very important to understand and keep in mind.

To awnser this question you have to look at QED (quantum electrodynamics).

Thanks for all your AWEsome work Richard P. Feynman

tech99

Gold Member
I was tutoring a student in an optics lesson the other day. We discussed the foundational concept, that light travels more slowly in a physical medium (such as air, water, or glass) than in vacuum. She asked, "Why? Because of friction?" and I said, "No, not friction," but then I had to admit, I didn't know what mechanism actually causes a light wave to slow down. It would seem more intuitive to me that a beam of light passing through a physical medium would lose energy / momentum (frequency).

But what causes it to slow down?
Maybe the medium stores energy? Like a transmission line.

nitsuj

As the photon exits the media to again propagate through a vacuum it immediately assumes speed c and it's original frequency. .

Not always. The wave can be changed so that it's speed is less than c when reentering "free space".

Drakkith

Staff Emeritus
2018 Award
This thread lay dormant, peacefully sleeping for two years until it was necro'd from the grave today (And this is after being necro'd once before). It's been three years since this thread had serious discussion and some of the posters in this thread are banned or no longer around. If anyone wants to discuss this topic they can start a new thread. Thread locked.

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