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Slowing down the speed of light in a medium

  1. Sep 30, 2011 #1

    Before I phrase my question, I'll just go through what I know so no unnecessary comments happen.

    I've read that the speed of light slows down in a medium because when a photon hits an electron the electron absorbs the energy of the photon and re-emits another photon.

    I also know that when an electron absorbs the energy of the photon it moves up a shell/energy band around the nucleus (I mean shell as in the electron setup 2,8,8 etc)

    Furthermore I have read that the electron then moves down a shell, back to it's original shell.

    My questions are the following:
    1)When an electron moves down a shell - is this when it re-emits a photon?
    2) How is it possible that an electron can move up a shell? I thought that the shells could only contain a specific number of electrons, hence the shells.
    3) Why are there shells anyway? I read somewhere else that the electrons are in a cloud around the atom, with no specific location and therefore why should they be in shells?
    4) If the electron has just gained the energy, does it move down again because it is unstable and cannot maintain this new energy? If so, why don't other electrons follow?
  2. jcsd
  3. Sep 30, 2011 #2
    You are confusing several different phenomena. The first effect (absorption), is where an electron in an atomic shell can only transition to certain energies, and can therefore only absorb certain frequencies of light. An electron that absorbs a photon transitions up in energy, but then typically transitions back down and looses its energy by knocking into other electrons. Thus light that is absorbed is usually lost to waste hear (unless you fine tune the system to encourage re-emission of a photon, such as in a laser). If you shine white light through a material and plot the spectrum that comes out the other sides, you generate an absorption spectrum. The dark lines are the frequencies of light that were absorbed and did not come out the other side, and they correspond to specific atomic electron transitions.

    Now there is a different effect (refraction) where the light is slowed down but is not absorbed. Yes, you can think of the light as being absorbed and re-emitted a moment later by a group of electrons so that the average light speed is slower. But this picture is misleading. This picture is not saying that individual photons are absorbed by individual electrons as the reason the light is slower. If that where so, only light at the resonant frequencies would slow down, but in reality, all frequencies slow down. If this picture is to be used at all to explain refraction, it should be used more as a classical picture where light is a wave that is absorbed by a large number of atoms in bulk and then re-emitted. The best picture of all though is that in refraction, the light never gets absorbed at all. Instead, it interacts with the electromagnetic field of the material so that it slows down. Think of it as like trying to swim through honey. You interact with the honey and you slow down. Except that light does not experience friction or viscosity, it experiences electromagnetic fields.

    Now to your questions:

    1) An electron can move down a shell by emitting a photon, but it can also do so by knocking into other electrons or atoms and loosing its energy to heat.

    2) An electron can only move up into a shell with an empty spot. That is why mostly only the outer electrons are involved in absorption of visible light.

    3) They are in shells, because the electron is a wave that can only take on certain shapes for certain situations.

    4) For an electron to move down to a lower energy atomic shell, that shell must have an empty spot. Once it moves down, the shell is filled, so that other electrons cannot follow it. Electrons transition when they are knocked by something else: another photon, another electron. Even "spontaneous" transitions are actually the electron getting knocked by a particle that fluctuates into existence from the vacuum. All of these processes are very predictable for a large group of electrons, so that we can calculate the average amount of time before an electron transitions down (it's lifetime).
  4. Sep 30, 2011 #3
    Fundamentals of optics (Jenkins and White) explains the "slowing down" as follows (and I like it):

  5. Sep 30, 2011 #4
    Thank you for your replies, especially chrisbaird.

    Please could you explain the part when you said that electrons can only take specific shapes for specific situations or provide me with a link to some resources?

    Thanks again
  6. Oct 3, 2011 #5
    Any introductory book, article, or website on quantum theory will get into electron wavefunction shapes. For example, http://en.wikipedia.org/wiki/Atomic_orbital" [Broken].
    Last edited by a moderator: May 5, 2017
  7. Oct 3, 2011 #6


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    Please start by reading the FAQ sub-forum in the General Physics forum.

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