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What is a reflection?

  1. Aug 27, 2007 #1
    How does light reflect? I am going to speculate below and maybe one of you all who have more experience in Physics can tell me if I am correct or give me a reference. I have an undergraduate degree in EE so you can talk to me as someone with a little college physics but (obviously by what I am going to write) not a whole lot.

    Here goes:

    A photon flies through space and is absorbed by an atom. This knocks a electron to a higher energy level. When the electron drops back it emits a photon at the frequency of the energy gap. The photons go off in all directions but it appears that the outgoing light ray reflects at the same angle as the incoming ray because of destructive interference of the out of phase rays. The energy in the gab determines the color (frequency). However, an intense enough light can cause electrons to jump to higher gaps causing the result to look white.

    How do mirrors work? Is it related to the large numbers of free electrons in the metal soup? And transparent medium like glass, do the photons go straight through or are they absorbed and retransmitted?

    This is not for any class or anything just curiosity.
  2. jcsd
  3. Aug 27, 2007 #2
    I think the usual language is that sometimes when the photon passes an atom, it may get scattered in any direction. Interference does determine the angle of aggregate reflection, but the atom isn't usually excited to a higher discreet energy level per say (most photons would carry the wrong energy amount), it is more just "perturbed" briefly (although the scattered light generally does have a phase shift/delay, and so it's perhaps debatable whether you can say it was "absorbed" briefly).
  4. Aug 27, 2007 #3


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    They are absorbed and re-emitted. The time taken for this explains why light goes slower in glass (ie. refractive index).
  5. Aug 28, 2007 #4
    Except that's not really true, is it? Atoms from materials with differing refractive indices will not take differing periods of time to scatter a photon, but will have differing probabilities of scattering a photon.
  6. Aug 28, 2007 #5
    There are two issues in this thread: Reflection as in a mirror, and transmission through a transparent medium.

    Mirror reflection requires free electrons in the material - metallic surfaces are good (a mirror can be made by silvering a glass surface), because a metal has a 'gas' of electrons which are free to move from atom to atom. This is why metals also conduct electricity. The reflection is caused by an interaction between the incoming light wave and a collective effect of the many electrons - the photon is not captured by a single atom.

    Reflection at a surface like from a windowpane is also an interaction between the light wave and a collective effect of the media - the sudden change in the refractive index causes a partial reflection.

    Transmission of a light wave through a transparent medium also does not involve absorption and emission of a photon by an atom. Again it is an interaction of the light wave and its surroundings - in this case the electrons cannot move from atom to atom easily, but can 'shift positions' a little in response to the passing wave - the index of refraction of the material is a measure of how strongly the environment can affect the light wave.

    Absorption and emission of a photon is statistical enough that the directions of the incoming and outgoing photons are in general uncorrelated.
  7. Aug 28, 2007 #6
    Thanks for the answers.
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