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FAQ - Do Photons Move Slower in a Solid Medium?

  1. Jul 4, 2008 #1
    FAQ - "Do Photons Move Slower in a Solid Medium?"

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

    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?
     
  2. jcsd
  3. Jul 4, 2008 #2
    Re: FAQ - "Do Photons Move Slower in a Solid Medium?"

    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 reference.]
     
  4. Jul 5, 2008 #3

    madmike159

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

    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.
     
  5. Jul 5, 2008 #4
    Re: FAQ - "Do Photons Move Slower in a Solid Medium?"

    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.
     
  6. Jul 7, 2008 #5
    Re: FAQ - "Do Photons Move Slower in a Solid Medium?"

    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 travelling 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 travelling through a vacuum?

    And again, how do we account for reflection?
     
  7. Jul 8, 2008 #6

    madmike159

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

    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.
     
  8. Jul 8, 2008 #7

    Cthugha

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

    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.

    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.
     
  9. Jul 9, 2008 #8
    Re: FAQ - "Do Photons Move Slower in a Solid Medium?"

    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 gonna be a lot of empty space before the photon reaches the next molecule.

    Point being, the "delay" must be pretty huge.
     
  10. Jul 9, 2008 #9
    Re: FAQ - "Do Photons Move Slower in a Solid Medium?"

    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).
     
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