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What does it mean, Electron absorbs a photon ?

  1. Jul 21, 2011 #1
    What does it mean, "Electron absorbs a photon" ?

    Hi Guys,

    I'm new to QED, and I was going through wikipedia link.

    I didn't understand what does it mean when they say an electron absorbs a photon.

    All I was taught in school, is that the process involves a shift in the energy level of an electron upon absorption of a photon of a specific energy (whose energy is equal to the difference in the energy levels).

    But I get some crazy thoughts.

    1) Does the mass of an electron increase?

    2) Does speed of an electron decrease, since it has gained mass?

    3) Once an electron absorbs a photon of specific frequency, and later when it emits one, it has to be of the same frequency?

    I'm not sure if that sounds stupid, but any help is appreciated.

  2. jcsd
  3. Jul 21, 2011 #2


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    Re: What does it mean, "Electron absorbs a photon" ?

    I don't think an isolated electron can absorb a photon, but it can scatter off one. (read Compton Scattering.) An electron in an atom or ion can absorb a photon by changing its energy level. In this case, the mean radius of the electron cloud increases, and the velocity decreases. The increased energy is stored in the electric field.
  4. Jul 21, 2011 #3
    Re: What does it mean, "Electron absorbs a photon" ?

    If its stored in the form of an Electric field, then does the charge configuration of the atom change? I'm thinking that, for electric field to change, either charge has to change, or distance has to change between two quantities. Here, when an atom absorbs a photon, then its radius will increase. According to coulomb's law, the electric field must decrease. I don't know if i'm thinking the right way.
  5. Jul 21, 2011 #4


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    Re: What does it mean, "Electron absorbs a photon" ?

    I was corrected by someone here on PF on something similar to this. Apparently the whole atom absorbs the energy of the photon, not just the electron. How this energy changes the atom depends on many variables and is probably at least a little bit random.

    If energy is imparted to one of the electrons, it's orbital will change to a higher energy orbital. The field itself from the electron doesn't change, but because of the position and energy are now different the electron affects the atom differently now.
  6. Jul 22, 2011 #5
    Re: What does it mean, "Electron absorbs a photon" ?

    If you mean just what is happening physically to each particle, I'm going to guess nobody knows exactly what it means...because I don't think anyone knows exactly what a photon or an electron is.

    But I can see relationships to other well accepted theories that provide some insights:

    One thing that makes it odd from a quantum mechanics perspective, is that it's an instantaneous, discontinuous energy transition for a bound electron. This puts it at the very heart of QM. Bound electrons are confined in some way.... such as an electron in an atom or a "particle" in a box or an electron stuck in a negative square potential well.....the wave function has the form of standing waves...it is quantized as is limited to certain energy levels. It behaves as a wave (distribution) rather than as a classically envisioned "particle".

    You can gain some likely insight by thinking about DeBroglie waves....reflecting the wave- particle (dual) nature of matter. From this perspective, photon absorption apparently changes the energy of the wave but not it's fundamental form of vibration...meaning the electron remains an electron...it retains its characteristics that make it an electron. This view fits with the hypothetical mathematics of string excitations....which ARE particles....amplitudes of string vibrations reflect energy and forms of vibrations reflect characteristics.
  7. Jul 22, 2011 #6
    Re: What does it mean, "Electron absorbs a photon" ?

  8. Jul 22, 2011 #7
    Re: What does it mean, "Electron absorbs a photon" ?

    It means that before certain process, there were an electron (probably bound to a nucleus) and a photon. Afterward, there is just an electron (and the nucleus). The energy and momentum of the electron + nucleus increase equal to the amount delivered by the photon.

    How does the photon vanish? Nobody knows.
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