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Photon Energy

  1. Sep 20, 2015 #1
    I was wondering about the amount of energy in Photons. Do they all have the same amount of energy? Or do they have a base amount which can increase depending on frequency or some other parameter? Also, I have read that photon photon collisions can yield positrons and electrons, both positrons and electrons have mass and thus energy via e=mc^2so does this interaction conserve energy?
     
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  3. Sep 20, 2015 #2

    mfb

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    Their energy is proportional to their frequency: E=hf. There is no "base amount".
    Every interaction conserves energy. The energy of the photons is converted to energy of electrons and positrons - the electron and positron masses are a part of this energy.
     
  4. Sep 20, 2015 #3
    Ok, so does that mean there is a minimum frequency for the electron/positron conversion? Also, if you decrease the frequency of a photon to near zero can we still detect it? And do photons exert radiation pressure proportional to their frequency?
     
  5. Sep 20, 2015 #4

    e.bar.goum

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    1. Yes. 1022 keV, which gives an electron/pair each with rest mass energy 511 keV. Further, to satisfy momentum conservation, this has to occur near a nucleus, so it takes some recoil.
    2. Depends on how sensitive your detection system is.
    3. Yes, photon momentum is given by p=hf/c
     
  6. Sep 20, 2015 #5

    mfb

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    Joe was asking about photon-photon collisions. Those are rare, but they don't need a nucleus.
    At some point a description via fields becomes more useful. We can detect fields that are changing extremely slowly, and even static fields. Single-photon detection at very low energies is extremely problematic.
     
  7. Sep 20, 2015 #6

    e.bar.goum

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    That'll teach me to read the thread properly. o:)
     
  8. Sep 20, 2015 #7
    I would imagine low energy detection would be problematic. Static fields is a concept I haven't heard of before, how do we sense them/create them?
     
  9. Sep 20, 2015 #8

    mfb

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    With a compass, for example. Or with a voltmeter for electric fields.
     
  10. Sep 20, 2015 #9
    Ok, separately that makes sense, but together... are there static electromagnetic fields?
     
  11. Sep 20, 2015 #10

    mfb

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    A battery in a magnetic field?
     
  12. Sep 20, 2015 #11
    Hmmm ok. Is it possible to induce photons into existance in a vacuum with stationary or oscillating electric and magnetic fields?
     
  13. Sep 20, 2015 #12
    A flourescent light tube does that I think.
    Well not a vacuum but a gas at very low density.
     
  14. Sep 21, 2015 #13

    Drakkith

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    Indeed. The photons are created entirely by the gas particles, not the vacuum itself.
     
  15. Sep 21, 2015 #14
    Variable electromagnetic field *is* light (photons). If you wave a magnet, it emits photons. Very low frequency, low energy radio waves, but still.

    Not very strong static electromagnetic field in a vacuum does not produce light.

    Ultra-strong static electromagnetic field can produce electron-positron pairs, even in vacuum, and accelerate them, which will make their fields non-static (they are accelerating) and thus emit photons.
     
  16. Sep 21, 2015 #15
    That is really cool. If you oscillated a single magnetically charged modlecule could you essentially create a low power laser which you could adjust the frequency?
     
  17. Sep 21, 2015 #16
    Ordinary radio transmitters do something similar - they emit their photons by moving charged particles (electrons) back and forth with the desired frequency.

    Laser is a device which emits great numbers of IR, visible, or UV photons with the same phase and polarization. (Radio-wave "laser" is called "maser").
     
  18. Sep 21, 2015 #17
    If vibrated in a fixed direction could you make a directional maser/laser without the use of optics?
     
  19. Sep 21, 2015 #18

    mfb

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    That's exactly what antennas do.

    Emission happens mainly orthogonal to the line of motion, however, to focus them you need a parabolic mirror or something similar.
     
  20. Sep 21, 2015 #19
    Hmm ok, that changes my understanding substantially, I always assumed the photons released by radio transmission was due to dropping electron orbitals. Thanks!
     
  21. Sep 21, 2015 #20

    mfb

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    That process is more typical for the emission of visible light (and some infrared and UV).
     
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