Why light slows in transparent media

In summary, Don Lincoln, a senior researcher at FermiLab, explains that light slows down in transparent media because the photons excite the electrons in the medium, adding to the wave. This does not result in secondary waves following the original one like an echo or sympathetic vibration, as they are coherent and do not bring or subtract energy. The electrons are not necessarily jumping to different orbitals, but rather the orbitals are slightly distorted by the fields of the incident wave. This was determined experimentally through the study of the Stark effect and Zeeman effect, which showed how other proposed explanations differ from what is observed.
  • #1
Sophrosyne
128
21
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?
 
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  • #2
Sophrosyne said:
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.
 
  • #3
trurle said:
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.
 
  • #4
Sophrosyne said:
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.
 
  • #5
Sophrosyne said:
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".
 

Related to Why light slows in transparent media

1. Why does light slow down in transparent media?

Light slows down in transparent media because the particles in the medium (such as glass or water) interact with the light waves, causing them to scatter and change direction. This interaction slows down the overall speed of the light.

2. How does the density of a transparent medium affect the speed of light?

The density of a transparent medium affects the speed of light because the more particles there are in the medium, the more interactions there will be between the light waves and the particles. This results in a slower speed of light.

3. Does the color of light affect its speed in transparent media?

Yes, the color (or wavelength) of light can affect its speed in transparent media. This is because different colors have different wavelengths, and these wavelengths can interact differently with the particles in the medium, resulting in variations in speed.

4. How does the refractive index of a transparent medium relate to the speed of light?

The refractive index of a transparent medium is a measure of how much the speed of light is reduced when passing through that medium. The higher the refractive index, the slower the speed of light will be in that medium.

5. Can light ever travel at the same speed in a transparent medium as it does in a vacuum?

No, light can never travel at the same speed in a transparent medium as it does in a vacuum. The speed of light in a vacuum is the maximum speed at which light can travel, and any interaction with particles in a medium will always result in a slower speed.

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