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Clarifying the rules of Reflection

  1. May 26, 2010 #1
    I have searched topics like reflection forum and I always seem to get different answers. I'm hoping to reconcile all of this, I've read FAQs, threads, wikipedia...

    To explain reflection I've heard these 3 explanations:

    1) Light is absorbed by an atom, then re-emitted at a later time randomly. My problem with this is that the light must have the specific quantum energy, so this does not explain all reflection...though maybe some? Also the atom doesn't necessarily have to give up the absorbed photon later.
    2) Light vibrates the atom as it passes *through* the atom, and thus the atom since it is electrically charged, when it vibrates it sends out a similar vibration, thus appearing to reflect. My problem with this is that perhaps this violates some sort of conservation of energy? Wouldn't this then mean that we have one outgoing photon that has done the vibrating of the atom, and then one reflected photon as a result, making two photons of the same energy (double the energy of what we had started with)?
    3) The photon directed at an atom is not of the proper quantum energy, and thus the atom acts as an impenetrable shield and the photon is smacked like a billiard ball in a Snell's law sort of way (assuming the atom is bound).

    I realize this topic is starting to be beaten to death but really, if anyone has done searching on this topic I think you'd also see the explanations are often at odds with each other.
  2. jcsd
  3. May 26, 2010 #2


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    (2) is the best answer. If you do an experiment using 1 photon at a time, each photon is detected as being either transmitted or reflected, with the probability of each given by the classical transmission and reflection coefficients.
  4. May 26, 2010 #3


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    Visual light is not typically absorbed by atoms (as in being coupled to the motion of atomic nuclei) but by their electrons. And the 'specific quantum energy' need not be very specific. Notably, in metals, these levels get smeared out to continuous bands.

    The electrons can absorb a photon (if it's the right energy) and then re-emit it. It's this process that becomes reflection and refraction. Energy is conserved during this.

    This just doesn't happen. If the photon is the 'wrong' energy, then it will just pass through it.
    (It can also possibly gain or lose energy, (changing the electronic state) which can be viewed as inelastic scattering, and then there's stimulated emission, and a whole bunch of other phenomena, but these are not related to reflection)
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