FAQ - Do Photons Move Slower in a Solid Medium?

[Usaf Moji]

FAQ - "Do Photons Move Slower in a Solid Medium?"

I have a question regarding this post in the FAQ's:

A common explanation that has been provided is that a photon moving through the material still moves at the speed of c, but when it encounters the atom of the material, it is absorbed by the atom via an atomic transition. After a very slight delay, a photon is then re-emitted. This explanation is incorrect and inconsistent with empirical observations. If this is what actually occurs, then the absorption spectrum will be discrete because atoms have only discrete energy states. Yet, in glass for example, we see almost the whole visible spectrum being transmitted with no discrete disruption in the measured speed. In fact, the index of refraction (which reflects the speed of light through that medium) varies continuously, rather than abruptly, with the frequency of light.

Secondly, if that assertion is true, then the index of refraction would ONLY depend on the type of atom in the material, and nothing else, since the atom is responsible for the absorption of the photon. Again, if this is true, then we see a problem when we apply this to carbon, let's say. The index of refraction of graphite and diamond are different from each other. Yet, both are made up of carbon atoms. In fact, if we look at graphite alone, the index of refraction is different along different crystal directions. Obviously, materials with identical atoms can have different index of refraction. So it points to the evidence that it may have nothing to do with an "atomic transition".

When atoms and molecules form a solid, they start to lose most of their individual identity and form a "collective behavior" with other atoms. It is as the result of this collective behavior that one obtains a metal, insulator, semiconductor, etc. Almost all of the properties of solids that we are familiar with are the results of the collective properties of the solid as a whole, not the properties of the individual atoms. The same applies to how a photon moves through a solid.

My question is, to what extent is the above applicable to photons moving through a liquid medium? Do the atoms and molecules that form a liquid also lose their individual identity and form a collective behaviour that governs the transmission of photons?

 

[cesiumfrog]

You have the mistaken impression that the answer to why "Photons Move Slower in [any] Medium" is primarily the result of "the collective properties of the [medium] as a whole, not the properties of the individual atoms". Quite simply, it is false (not just naive) that "disturbance of the lattice [..] is the origin of the apparent slowdown of the light speed in the material".

Light is slowed even in a dilute gas. Slowing of light requires only one property: that the medium has some small ability to be electrically polarised. All atoms have this property individually (a small perturbation in electric field causes a small perturbation in electron distribution around the nucleus, even without actually "exciting" the electron). [See https://www.physicsforums.com/showthread.php?t=173745".]

 

[madmike159]

The speed of light is about half in diamond. Think back to the glass block experiments where the light was refracted because it moved slower in the glass than in air.

 

[peter0302]

So if diffraction is not an atom being absorbed and reemitted, what is reflection? Is the reflected photon the _same_ photon that hit the mirror? I assume the answer is no - in which case how does the idea that the atoms in the mirror emit a new photon jive with the FAQ's point that the electrons have discrete absorption energy states.

 

[peter0302]

I have other difficulties accepting the FAQ's explanation. Photons are point particles - i.e. they're infinitessimally small. Yet atoms -including atoms in a solid - are 99.9999...% empty space. The photons - most of that time - therefore should be traveling at the speed of light. Therefore, to account for the dramatic speed reduction resulting from the interaction with matter, that mass must be having a dramatic effect on the photons. What is causing that effect? Is it the EM field of the electrons? Can EM fields slow down photons traveling through a vacuum?

And again, how do we account for reflection?

 

[madmike159]

The photon hits a surface and its energy is asorbed and re-emmited when electrons loose energy. A photon isn't really a point particle because they have no boundaries, but that means they have no size.

 

[Cthugha]

The photon hits a surface and its energy is asorbed and re-emmited when electrons loose energy.

Ehm...no. This is as wrong as it gets in this case. The slowing of light in a medium does NOT need absorption. Please...at least read the FAQ before answering to topics concerning the FAQ.

I have other difficulties accepting the FAQ's explanation. Photons are point particles - i.e. they're infinitessimally small. Yet atoms -including atoms in a solid - are 99.9999...% empty space. The photons - most of that time - therefore should be traveling at the speed of light.

This is very much the old fashioned Rutherford point of view. However, from a quantum point of view, you have to keep in mind, that you need wave functions to describe the situation accurately instead of point particles. The explanation cesiumfrog gave, is rather correct here.

 

[peter0302]

This is very much the old fashioned Rutherford point of view. However, from a quantum point of view, you have to keep in mind, that you need wave functions to describe the situation accurately instead of point particles. The explanation cesiumfrog gave, is rather correct here.

Sure, but let's say we're talking about hydrogen gas. There's always going to be 2, and only 2, electrons per molecule. The photon will interact with one, and maybe the other, and then that's it - doesn't matter what the wavefunction of those electrons is - it will have collapsed. Then there's going to be a lot of empty space before the photon reaches the next molecule.

Point being, the "delay" must be pretty huge.

 

[lightarrow]

So if diffraction is not an atom being absorbed and reemitted, what is reflection? Is the reflected photon the _same_ photon that hit the mirror? I assume the answer is no - in which case how does the idea that the atoms in the mirror emit a new photon jive with the FAQ's point that the electrons have discrete absorption energy states.

If you are talking about reflection by a mirror, then it's metallic reflection; in this case energy states are not discrete, but continuous (conduction band).