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Reflection or absorption?

  1. Apr 18, 2006 #1
    How do an atom "know" when to absorb a photon, and when not to? It seems like an atom absorbs all photons that either has energy that corresponds to the difference between two energylevels, or has any type of energy at all in which case the photon energy helps speed up the atoms oscillations. And, in some cases, it reflects the photons. I'm really confused. Why are some photons reflected?
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  3. Apr 18, 2006 #2


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    Energy has to be conserved, exactly. So the atom can only aborb a photon of a given energy if it is capable of "internalizing" that energy by passing to a higher energy state. Since quantum reality only allows it to do that for certain energies, it can only absorb photons of those energies.
  4. Apr 18, 2006 #3
    But an ideal blackbody absorbs radiation of all wavelengths. How is this compatible with the above statement?
  5. Apr 18, 2006 #4


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    A blackbody is not a single atom with a relatively small number of energy states, but a complex system of bazillions of atoms.
  6. Apr 18, 2006 #5


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    Also, photons are not reflected by individual atoms, but by a large ensemble of them.
  7. Apr 18, 2006 #6
    So an ideal blackbody is a collection of atoms which together are able to absorb photons of all wavelengths since it is made of such a variety of elements?

    Photon-reflection in a crystal were used to examine x-rays in 1912 by W.L. Bragg. How do you mean reflection is done by a group of atoms, instead of a single atom?
  8. Apr 18, 2006 #7


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    You may want to start by reading our FAQ in this section of PF first. It will answer your "absorption" part and why your original post isn't quite right when you were focusing on "atoms".

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