Why light slows in transparent media

[Sophrosyne]

I was watching this video by Don Lincoln, one of the senior researchers at FermiLab, on the the reason light slows down in transparent media (air, water, glass, plastic, etc...).



He explains that the photons excite the electrons in the medium, which in turn add to the wave (or at least that's what I understand of his explanation).

My question is:

1) Why would this slow down the original light wave? Why not have the secondary waves follow the original, like an echo or like a sympathetic vibration?

2) How is this excitation in those electrons occurring? Are they getting bumped up to different orbitals, and then falling back down again, much the same way we see color? Is it that these transparent materials have orbitals that transmit in ALL the different wavelengths of the original light so that none of it is absorbed by the material?

 

[trurle]

Summary: Why does light slow down in transparent media?

I was watching this video by Don Lincoln, one of the senior researchers at FermiLab, on the the reason light slows down in transparent media (air, water, glass, plastic, etc...).



He explains that the photons excite the electrons in the medium, which in turn add to the wave (or at least that's what I understand of his explanation).

My question is:

1) Why would this slow down the original light wave? Why not have the secondary waves follow the original, like an echo or like a sympathetic vibration?

2) How is this excitation in those electrons occurring? Are they getting bumped up to different orbitals, and then falling back down again, much the same way we see color? Is it that these transparent materials have orbitals that transmit in ALL the different wavelengths of the original light so that none of it is absorbed by the material?

1) Because the secondary waves are coherent with initial one and do not bring or subtract energy (the magnetic forces on electron are 90 degrees to the direction of its motion, and therefore no work is done, while electrical forces are well averaged out along orbit). Therefore, secondary waves are not separable from incident wave.
2) No. If electrons are jumping to another orbitals, it is typically absorption. Some experimental setups with ultra-slow propagation of light or even "temporary light stopping" do involve electrons jumping to another orbitals, but in typical transparent media the orbitals are just slightly distorted by the fields of incident wave.

 

[Sophrosyne]

1) Because the secondary waves are coherent with initial one and do not bring or subtract energy (the magnetic forces on electron are 90 degrees to the direction of its motion, and therefore no work is done, while electrical forces are well averaged out along orbit). Therefore, secondary waves are not separable from incident wave.
2) No. If electrons are jumping to another orbitals, it is typically absorption. Some experimental setups with ultra-slow propagation of light or even "temporary light stopping" do involve electrons jumping to another orbitals, but in typical transparent media the orbitals are just slightly distorted by the fields of incident wave.

Thank you for that.

I guess the only question I have then is about "distortion of orbitals"- since I thought they had to stay strictly quantized. But I guess I can see how the energy of the incident light wave would be subtracted from the quantized potential energy of the electron from its parent nucleus, and that way it would still maintain the baseline quantized potential energy of the orbital while still being able to be distorted in a non-quantized way.

The other question is: how in the world was this determined experimentally? Or is it just "common sense" to physicists? Because it seems like verifying that explanation experimentally would require some very fancy experimental set-up.

 

[PAllen]

Thank you for that.

I guess the only question I have then is about "distortion of orbitals"- since I thought they had to stay strictly quantized. But I guess I can see how the energy of the incident light wave would be subtracted from the quantized potential energy of the electron from its parent nucleus, and that way it would still maintain the baseline quantized potential energy of the orbital while still being able to be distorted in a non-quantized way.

The other question is: how in the world was this determined experimentally? Or is it just "common sense" to physicists? Because it seems like verifying that explanation experimentally would require some very fancy experimental set-up.

The video itself shows how the predictions of other proposed explanations differ radically from what is observed.

 

[trurle]

Thank you for that.

I guess the only question I have then is about "distortion of orbitals"- since I thought they had to stay strictly quantized.

The other question is: how in the world was this determined experimentally?

Please search for "Stark effect" and "Zeeman effect".