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Photons are produced at random?

  1. Nov 16, 2006 #1
    Consider a hydrogen atom. A photon strikes it at some random angle. The hyrogen's electron absorbs the photon. The electron becomes excited and its orbital increases. The electron deexcites and emits a photon.

    Question: Is the direction of the emitted photon a random direction. Or is there a correlation with the emitted photon's direction with the original incident photon's direction?
  2. jcsd
  3. Nov 16, 2006 #2


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    You can answer your own question if you consider conservation of momentum. (A photon of energy E has momentum |p| = E/c.)
  4. Nov 18, 2006 #3


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    Both can occur. It if there is a correlation, we talk about a coherent process, if not, we talk about an incoherent process. Usually the "absorption-re-emission" process is incoherent.
  5. Nov 18, 2006 #4
    That seemed reasonable at first (I can see through water), but then I started thinking about absorption spectra. If I hit a sample with photons of an appropriate energy to excite the atoms, and assume the atom will emit the exact same frequency, then obviously in the atom's rest frame it cannot have altered the photon's direction. But without scattering, I shouldn't be able to measure absorption spectra.

    Is this actually a mechanism that prohibits certain absorption lines, or was my assumption wrong?
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