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Photons = energy? no?

  1. May 14, 2008 #1
    Why can't photons be the same as energy? How are they different?
     
  2. jcsd
  3. May 14, 2008 #2

    Danger

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    Welcome to PF, BW.
    Photons are the bosuns (carrier particles) for electromagnetic energy. While they are not energy themselves, they do transport it from place to place.
    That's the best that I can do by way of explanation, given my own limited education, but others will be along shortly who can do a better job of it.
     
  4. May 15, 2008 #3

    Hootenanny

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    Energy is a property of a particle/system. Saying photons are energy is like saying electrons are charge.
     
  5. May 15, 2008 #4
    Very well put, easily understood descriptions!

    I love it when longwinded dogma is avoided on subjects like this.




    (to help prevent 'longwinded' responses to my response, I replaced "definitions" with descriptions. lol)
     
  6. May 15, 2008 #5
    Another good way to put it would be to say that photons have properties other than their energy. In fact, photons also carry momentum and angular momentum.
     
  7. May 15, 2008 #6
    Is it possible to have a photon with no energy left to tranfer?
    Would it behave differently?
     
  8. May 15, 2008 #7

    rcgldr

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    If photons have momentum, when an electron captures a photon, does it make any difference in the electrons energy or path after capture depending on the direction the photon was traveling before it was captured by the electron?
     
  9. May 15, 2008 #8

    dst

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    A man called Heisenberg asked that around a century earlier. He was never certain as to what would happen :rolleyes:.
     
  10. May 19, 2008 #9

    rcgldr

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    If it's unknown, then why do so many people state that photons have momentum? Is there any other way that photons could exhibit momentum properties?
     
  11. May 19, 2008 #10

    Hootenanny

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    In the analysis of Compton Scattering, the photon is assumed to be a particle that has a momentum determined by the de Broglie hypothesis.
     
  12. May 19, 2008 #11

    Dale

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    Yes. Energy and momentum are conserved.
     
  13. May 19, 2008 #12

    Mentz114

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    When an atom absorbs or emits a photon, momentum is interchanged. This was first explicitly shown by Einstein in a 1916 paper.

    He shows that when matter and radiation interact, the momentum interchange is necessary to get the Planck radiation curve.
     
  14. May 19, 2008 #13

    rcgldr

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    OK, but don't electrons have discreet energy states? So if an electron captures a photo, and jumps up to the next energy state, regardless of the direction of the photon, where does the momentum change due to the direction of the photon go?

    On a related issue, what determines if a photon will be captured and released as if reflected (mirror mode) as opposed to released with the same phase and direction (laser mode)?
     
  15. May 19, 2008 #14

    Dale

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    Electrons only have discrete energy states when they are in atomic orbitals. In that case the momentum "goes" into the whole atom.
     
  16. May 20, 2008 #15

    Mentz114

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    It is as DaleSpam says. This is the process by which atoms can be cooled by a carefully tuned laser 'molasses' beam.

    The beam is tuned to just below the frequency of an absorption line, so only atoms moving towards the source of the beam will absorb, and therefore slow down. The re-emisson of the photon is random, and the overall effect is to slow down the atoms.
     
  17. May 20, 2008 #16
    Ha ha! kudos on this one dst :cool:
     
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