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How fast must a magnet go to make visible light?

  1. Oct 29, 2008 #1
    If I get this whole “Moving magnet and conductor problem” business at all, one frame’s electricity is another frame’s EM wave. So moving a magnet causes an electromagnetic wave, AKA: photon. I assume the faster you move it, the higher the energy of that wave/photon. (Maybe you have to vibrate it to get a frequency. Maybe acceleration is the key.) So how fast do you have to move it, vibrate it or accelerate it to get a photon in the visible spectrum out of it?

  2. jcsd
  3. Oct 29, 2008 #2
    429 THz (red light at 700 nm).
  4. Oct 29, 2008 #3
    You may well "get it" but it should be stated "one frame's electric field is another frame's magnetic field"....each are different aspects of an electromagnetic wave...each will induce the same current...

    I just noticed Wiki says it this way:
    which seems clear.

    I'll be interested to see explanations to this and the rest of your question(s)...
  5. Oct 31, 2008 #4
    A photon's energy is proportional to its frequency. About a wave, its energy is the number of photons times the energy of a single photon.
  6. Oct 31, 2008 #5
    How does changing the speed of the magnet change the energy of the EM wave it generates?? The photon (or wave) always moves at the speed of light regardless of the velocity of the source.
    or to say it another way: if the magnet is held stationary and I approach the magnet at some velocity, the apparent frequency increases but the energy of the wave I don't think posssibly could.

    Note: see my post below for a revision of this idea....special relativity means frames of reference matter.
    Last edited: Nov 1, 2008
  7. Oct 31, 2008 #6
    I think you are right. The energy of the wave is given only by the Poynting vector S=E x H. So, in my opinion there's no dependence on the velocity.
  8. Oct 31, 2008 #7
    If you read my first post you notice I was talking about frequency (even the OP auto-suggested this fact). When you move a magnet in an alternating way, it generates an electromagnetic wave which has the same frequency; the amplitude of the wave, fixed the frequency, depends on the amplitude of the magnet's oscillation. The photon's energy depends only on its frequency, the amplitude of the wave depends on the number of photons. You can compute the energy that flows through a unit area in the unit time in two ways: classically and quantistically. In the first case you compute Poyinting's vector, in the second you compute a photon's energy and the number of photons traversing that area in the unit time and then multipying the two values.
  9. Nov 1, 2008 #8
    Here's an explanation from


  10. Nov 6, 2008 #9
    Nope. Moving a magnet creates a De Broglie wave but no photon. The De Broglie wave is not a true wave but more like a wake. It's not a traveling wave and therefore it's not accompanied by a photon. But you're right about the frequency part as it concerns the De Broglie wave.
  11. Nov 6, 2008 #10

    Jonathan Scott

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    Gold Member

    Under what limitations do you expect moving a magnet to produce only an "empty" wave? Constant acceleration?

    If I waggle a magnet around, I'm sure that can generate an electromagnetic wave which could transfer energy to an aerial.
  12. Nov 7, 2008 #11
    Do you mean by "empty" wave one that isn't associated with a photon and that isn't self-propelled? (i.e. non-radiative)

    If so, that's an excellent question that far too many researchers have overlooked IMO. That's really the physical end of QM. It's not quite as simple as the lack of acceleration or constant acceleration. There's a very interesting paper by the late Dr. Herman Haus in which he shows that the make or break condition for far field radiation is the existence of frequency components within the Fourier transform of the charge trajectory which are synchronous with the speed of light.

    Haus H. A. "On the radiation from point charges", American Journal of Physics, 54,
    (1986), pp. 1126-1129.

    Related investigations have been documented by Tyler Abbott, David Griffiths, G. Goedecke and Philip Pearle.
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