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Absorption of Light and Energy Conservation

  1. Nov 30, 2007 #1
    Hello everybody. I apologize if this question is a little trivial compared to alot of the threads posted, but I am an 8th grade science physical science teacher and am a little confused about some of the material.

    From what I understand, if an atom absorbs a photon of energy E, then an electron will jump to a higher energy level. The change in potential energy that results from the EM field will be equal to the energy of the absorbed photon. All this I got from my understanding of spectral lines. Where I am confused is that I know the photon also has momemntum which must be conserved. If the photon transfers is momentum to the atom, the atom must have some Kinetic energy, which on a larger scal will translate to an increase in thermal energy. This seems to violate energy conservation. I know I must be missing something....what is it?!?!

    Thanks for the help Y'all!
  2. jcsd
  3. Nov 30, 2007 #2


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    The atom does "recoil". However, you need to compare the momentum of a "typical" photon that is involved in such absorption versus the mass of the atom. For example, you don't see the earth recoiling when raindrops hit its surface, do you? Even a larger comet hitting the earth doesn't cause it to recoil much.

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