Photons through transparent materials, what's going on?

[fluidistic]

I have read, heard and seen texts and youtube videos about the slowing down of light through matter, and also about why some materials are transparent. I am satisfied with the explanation of the slowing down of photons through matter, but not the explanation of why materials are transparent. I would therefore like to know, exactly, what is going on with photons when they pass through a transparent material.

The usual explanation of the slowing down of photons through matter is that they make the electrons in the material oscillate (because they react to the EM field of the photon "beam"), creating themselves an EM field that adds up with the incident one, inside the material. In other words the photons become polaritons inside the material, i.e. massive quasiparticles explaining why they go slower than light. And that there is no real need to go as deep as QM, because a simple classical treatment seems to do the job too. If one adds up the incoming EM field with the one produced by the oscillating electrons, one gets a total field whose group velocity is lesser than the speed of light in vacuum. Job done. So far so good, I can buy that explanation.

The usual explanation of why some materials are transparent is that the incoming photons have not enough energy to excite electrons from a band to the next one. So no photon "collides" with electrons, they do not get absorbed. This seems to indicate (to me at least), that the photons are unable to interact with the electrons. If that was really the case, then light shouldn't slow down through transparent materials, but that's totally wrong.

So I guess that for transparent materials, the usual explanations of why photons slow down through matter still applies, but on top of it, the photons are unable to excite electrons to higher energy bands or better to say, higher energy states. But I'm not entirely sure that's correct.

Can someone write down once and for all a description of what's going on regarding photons/EM fields inside a transparent material. It can involve QFT, QED, QM, solid state physics and condensed matter.

 

[Henryk]

Photons interact with electrons, always. The interaction is between the charge of the electrons and the electric field of the photon (protons are charged too, but their mass is 1836 times greater, the interaction is proportionally smaller). A simplified picture of interaction of a photon with a single electron is like follow. There is an electron at certain energy within a piece of material. Here comes a photon of energy $h\nu$

 

[Henryk]

Sorry, clicked on the wrong button, let me continue.
An incoming photon raises the energy of the electron by its own energy. Now, everything depends on the properties of the material, specifically, the electron energy spectrum. If there is an allowed (and empty) electron state of energy equal the initial electron energy plus photon energy, the electron can (doesn't have to) make a transition to that state and the photon is annihilated.
If, however, there is nowhere for the electron to go, the electron falls back to it's original state, (that is the transition to the upper state was virtual, in the long run, energy must be conserved). What happens to the photon? It either continues in the original direction by phase shifted by the process (that's what gives you dispersion) or changes direction, that is becomes scattered. After all, the sky is blue and the sunsets are displays in all shades of reddish-orange.

 

[Xforce]

Even though the light seems to slow down when traveling through a transparent medium, but the speed of light between atoms are never changed, still 299792458 meters/second. The ratio of light speed in medium or light speed in vacuum or, the ratio of time between the absorb-emission plus the time light travel between atoms and the time light travels between atoms, are called a refractive index. Everyone should know that. But what is the negative refractive index in some materials such as plasma?

 

[DrClaude]

Have you read @ZapperZ's excellent Insight on the topic?
https://www.physicsforums.com/insights/do-photons-move-slower-in-a-solid-medium/

 

[Lord Jestocost]

There is absolutely no need to discuss light dispersion and absorption within the framework of the photon picture. I recommend to thoroughly read Richard Feynman’s lecture “The Origin of the Refractive Index“ which addresses these questions in a fundamental and elegant way: http://www.feynmanlectures.caltech.edu/I_31.html

 

[fluidistic]

Have you read @ZapperZ's excellent Insight on the topic?
https://www.physicsforums.com/insights/do-photons-move-slower-in-a-solid-medium/

Is that question asked to me or to @Xforce who seems to have no clue whatsoever on the matter?
I have read ZZ's article, and also the relevant remarks at the bottom, by Dr. Du and Daz. At visible spectrum incident light frequencies, the electrons matter much more than phonons, apparently, with regards to the explanation of why a material is transparent.

There is absolutely no need to discuss light dispersion and absorption within the framework of the photon picture. I recommend to thoroughly read Richard Feynman’s lecture “The Origin of the Refractive Index“ which addresses these questions in a fundamental and elegant way: http://www.feynmanlectures.caltech.edu/I_31.html

Thank you, I will have a look.

Edit: I just had a quick look at Feynman's explanation, but it doesn't match what I am seeking (as described in my first post). I know that from a classical viewpoint, the index of refraction has a real and imaginary part, and that the latter gives information on the absorption. Essentially what I understand from Feynman's article is that "because there is an imaginary part in the index of refraction, calculations yield an attenuation of the wave in the material", which is obviously true but says absolutely nothing about why the index of refraction has an imaginary part and how we could "guess" numerical values of it based on the material (glass, a metal, etc.) or at least from band theory, solid state physics, QFT, QED, etc.

 

[Lord Jestocost]

...but says absolutely nothing about why the index of refraction has an imaginary part

Within the classical Lorentz oscillator model for absorption and dispersion, the imaginary part of the refractive index results - so to speak - from the „damping“ of the electron motion.

 

[snorkack]

Does a laser also have imaginary part of refractive index? Because in a laser, the amplitude of wave increases rather than decrease or stay constant.

 

[Cthugha]

Essentially what I understand from Feynman's article is that "because there is an imaginary part in the index of refraction, calculations yield an attenuation of the wave in the material", which is obviously true but says absolutely nothing about why the index of refraction has an imaginary part and how we could "guess" numerical values of it based on the material (glass, a metal, etc.) or at least from band theory, solid state physics, QFT, QED, etc.

I am not sure I get your problem. In the most basic (and of course a bit too pedagogical) treatment possible, the physics of the refractive index is essentially already covered by the driven harmonic oscillator or driven dipole antenna. If you revisit the driven harmonic oscillator, you will find that you get a lot of energy into the system at or close to resonance, you get very little energy into the system when the driving frequency is below the resonance frequency and you get pretty much no energy into the system AND a pi phase shift of the response above the resonance frequency. This more or less already contains all you need. The absorption close to resonance is there and the phase shift of the driven system with respect to the driving force is also already there. A real material now consists of several resonances driven simultaneously, which just makes things a bit more ugly.

 

[Drakkith]

ven though the light seems to slow down when traveling through a transparent medium, but the speed of light between atoms are never changed, still 299792458 meters/second.

The space between atoms is less than the wavelength of even most x-rays, so I think the idea that light travels between atoms without them interacting with the light during the travel is fundamentally flawed. Instead I'm betting there is a lot of near-field effects that have to be taken into account.

 

[DrDu]

One of the classic papers on how to calculate the dielectric function of a material is Lindhard's paper on the dielectric properties of the electron gas:

http://gymarkiv.sdu.dk/MFM/kdvs/mfm 20-29/mfm-28-8.pdf

 

[PhilDSP]

It could also be worth looking at the Ewald-Oseen Extinction Theorem. This describes what is going on in terms of the fields and shows how the electrons in the transparent material or media are set into motion or oscillation. A recursive effect is set up between the moving electrons and the additional fields that movement generates.

https://arxiv.org/abs/1507.05234

 

[fluidistic]

It could also be worth looking at the Ewald-Oseen Extinction Theorem. This describes what is going on in terms of the fields and shows how the electrons in the transparent material or media are set into motion or oscillation. A recursive effect is set up between the moving electrons and the additional fields that movement generates.

https://arxiv.org/abs/1507.05234

Interestingly I've got to know about that theorem since I opened this thread, from a random physicist on youtube who claimed that the theorem is proven in Born & Wolf's "Principles of Optics" book. And indeed it (partly) is.

 

[cmb]

I have similar misunderstandings to the OP. Let me say what my confusions are.

So we have this packet of energy, a photon, whizzing through a vacuum and it comes across the surface of a piece of glass 1cm thick.

As it enters, does it interact with electrons in every atom along its path, some of them, or very few?

Is there a way to determine how many atoms in 1cm of glass a photon of 500nm interacts with? Is that what defines 'its' speed through a material.

Also, does this mean a photon cannot pass 'through' a transparent material, or does it only get through by repeated absorption and then recreation as a new photon?

If it is constantly absorbed and then re-emitted then it isn't really the same photon. In which case how does it stay in phase, and with all this fancy entanglement science how can entangled photons pass through a transparent material of an optical device without being absorbed and re-emitted, thus breaking the quantum entanglement?

In fact, even more basic is how does a photon remain heading in the same direction if it is constantly being absorbed and re-emitted by the electrons of atoms along its way?

It seems a bit fanciful that photons interact with electrons in this way, but I can't see the alternative description of it?

 

[Drakkith]

So we have this packet of energy, a photon, whizzing through a vacuum and it comes across the surface of a piece of glass 1cm thick.

No. This is already wrong. You cannot model light as photons flying through space like they are tiny billiard balls. A photon does move through space along a single defined path. It is better described as the quanta of interaction between an electromagnetic wave and either a charged particle or a field.

As it enters, does it interact with electrons in every atom along its path, some of them, or very few?

The electromagnetic wave interacts with many things all at once. You get a great deal of near-field effects along with other effects that are purely described by quantum field theory and are essentially impossible to describe using ordinary language. That's really the problem here. Quantum theory is so unlike our everyday experience that we can't even describe it adequately using anything but math.

In fact, even more basic is how does a photon remain heading in the same direction if it is constantly being absorbed and re-emitted by the electrons of atoms along its way?

It's not getting absorbed and re-emitted. The way the EM wave interacts with a large number of charged particles in a small area is very complicated. Not only that, it's also probabilistic, meaning that what happens each time a very low amplitude (energy of a single quanta) EM wave passes through cannot be predicted in advance. We can only say that we have X chance of thing A happening, Y chance of thing B happening, etc. You might have a photon scatter off of a molecules, or the EM wave might pass all the way through without depositing any energy, or it might get completely absorbed, or it might do a combination of several things. Even when it passes all the way through you have all sorts of things going on whose net effects are that the wave passes through without losing energy.

When talking about a high-amplitude wave things are worse, because you can have all of those things happen at the same time to varying degrees. The wave can partially pass through, lose some energy to scattering, lose some energy to absorption, etc.

 

[cmb]

No. This is already wrong. You cannot model light as photons flying through space ...The way the EM wave interacts with a large number of charged particles in a small area is very complicated.

So what of quantum entanglement? I thought this relates to individual photons, and if they interact then they lose entanglement?

Or more mundane, how does a photon multiplier work? I thought individual photons passed into a chamber with photoelectric parts and held under electric fields to accelerate and multiply any electrons that come off?

https://en.wikipedia.org/wiki/Photomultiplier_tube

 

[Drakkith]

So what of quantum entanglement? I thought this relates to individual photons, and if they interact then they lose entanglement?

I'm not saying photons don't exist, I'm saying you can't separate a photon from its EM wave. They are two parts of the same thing. One just emphasizes the wave behavior while the other emphasizes the way energy, momentum, and other properties are transferred from the wave to an object.

Or more mundane, how does a photon multiplier work? I thought individual photons passed into a chamber with photoelectric parts and held under electric fields to accelerate and multiply any electrons that come off?

That's right. But just remember that an 'individual photon' really means a quanta of energy/momentum/other properties that an EM wave has.

 

[cmb]

I am still very unclear on that. If you turn down the light level until you get one photon of light per second passing through 1cm of glass, does it interact with the glass or not, and if it does then does it lose entanglement if it is so entangled with another photon elsewhere?

 

[Drakkith]

If you turn down the light level until you get one photon of light per second passing through 1cm of glass, does it interact with the glass or not

Of course. It's just that the interaction isn't easy to describe.

and if it does then does it lose entanglement if it is so entangled with another photon elsewhere?

It might depending on what happens during the interaction.

 

[Cthugha]

I am still very unclear on that. If you turn down the light level until you get one photon of light per second passing through 1cm of glass, does it interact with the glass or not,

Please note that photons are not perfectly localizable point particles. Accordingly, it will interact with the crystal exactly like a light pulse of a spatial extension corresponding to the coherence length of the photon and a field strength scaled down to the field of a single photon would do. In other words: There is nothing additional you gain by using the photon picture. The behavior in a transparent medium does not change from the picture in classical optics. This is just the electromagnetic field pulling and pushing the charges or dipoles in the material around coherently. The physics is not really different from e.g. driving a harmonic oscillator (or a resonant circuit) far from resonance. Far above resonance you get a significant phase response, but that does not mean that you deposit all the energy in the system.

and if it does then does it lose entanglement if it is so entangled with another photon elsewhere?

If you are in the transparent region of a linear material: No, entanglement will stay intact. Otherwise, photon entanglement would be impossible to study. Pretty much every experiment requires lenses or polarization optics.

 

[Vanadium 50]

The whole concept "a photon enters the medium and slows down" is as wrong as wrong can be.

It assumes that a photon is a particle that can be identified ("this photon enters the medium but that one doesn't") and that it somehow maintains this identity inside the medium ("this photon is the same as that one"). Neither is true in QM. Furthermore, it makes the implicit assumption that "normal" or "regular" photons are the ones that propagate in vacuum and somehow the material changes them. That might only be one degree of wrong.

Let's go back to the beginning. You start with Maxwell's equations in free space, find that one of the solutions would be EM waves propagating at c, quantize this theory and now these waves become quantum objects propagating at c which we call photons.

In media, you start with Maxwell's equations in media (i.e. D and H fields are no longer identical to E and B fields), and find that one of the solutions would be EM waves propagating at v < c. Turn the quantization crank and again these waves become quantum objects propagating at v < c. These are photons too.

That is the key observation, and I think I want to say it again. These are photons too.

What happens when a photon enters a medium is that the energy and momentum excites the medium, and this excitation can take many different forms. If it excites a photon in the medium with similar E and p, we say the material is transparent.

"This is BS!" I hear some of you think. "Between atoms there is vacuum, and all this hogwash about D and H fields is only appropriate for bulk materials. I want to know how my photon in the vacuum between atoms can possibly affected by those atoms." Light wavelengths are around 5000 angstroms, and atoms are about an angstrom, so there are tens of millions of atoms responding at any instant to the EM fields of the wave. The model of continuous media simply fits better than the "atoms-and-vacuum" picture .

 

[Fred Wright]

I would like to make a comment which is slightly off topic but which may be of interest in trying to visualize the propagation of light through transparent media. In 1967 the Russian, Victor Veselago, wrote a remarkable paper in which he showed that if one could find or construct a media in which both the permittivity and the permeability of D and H obtain negative values, then the material would transmit light with the phase velocity anti-parallel to the Poynting vector (i.e. a backward propagating wave). Veselago showed the stunning implications of this for geometrical optics. In 2000 my former physics teacher, Shelly Schultz, made the first experimental observation of this effect at microwave frequencies by constructing a three dimensional lattice, called metamaterial,