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Quanta of Light

  1. Mar 8, 2014 #1
    Trying to understand small increments of light.

    From a particle perspective, a photon is the smallest increment, dependent on frequency as E=hf.

    From Maxwell's wave perspective, light propagates as a result of the energy passing between the E field and the B field. The rate at which this occurs gives the frequency.

    So from a wave perspective, how short can a wave of light be? Less than one wavelength?

    If I have a source, a radio transmitter, that emits 10 1/4 lambda of light, and 10 lambda is received/filtered/reflected, can the 1/4 thats left when the filter is removed still propagate?
    Last edited: Mar 8, 2014
  2. jcsd
  3. Mar 8, 2014 #2


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    Think some more about classical E&M before you worry about photons.

    Nope, that's not the way an EM wave works. The energy does not pass from E to B and back again. E and B are in phase. At the crest of the wave, E and B are both maximum simultaneously. Ninety degrees later, at the node, they are both zero simultaneously. The energy density is E2 + B2. That means there is plenty of energy at the crest, shared by E and B in equal amounts, and none at all at the node.

    A short wave pulse cannot be a pure sine wave. If you cut off a sine wave into a short pulse, the wave will no longer be pure, it will contain high frequencies. Frequencies so high that their wavelength is comparable to the pulse length.
  4. Mar 8, 2014 #3
    So if you cut monochromatic laser light off it will change frequency?
  5. Mar 8, 2014 #4
    Example, an extremely low frequency em wave, 0.1hz, if you take a 2.5 second burst is it not photons present in a 1/4 wavelength of a sinusoidal charge/magnetic field?
  6. Mar 8, 2014 #5
    laser light^
  7. Mar 8, 2014 #6
    I'm thinking about it in terms of a particle accelerator. If I use the 1/4 lambda above can I aim that at a proton and get it to accelerate due to the charge wave?
  8. Mar 8, 2014 #7


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    Yes, just as the shape of water waves will change if you do something to disturb their propagation. This is one of the reasons that Bill_K is advising you to understand classical E&M before you try thinking in terms of photons.
  9. Mar 8, 2014 #8
    Alright. So can I read any math that will describe this? the limit of the existence of the wave? The other effect I was interested in is the faraday effect... how sharply/irregularly can I twist a linearly polarised wave and keep it as propagating light?
  10. Mar 8, 2014 #9
    Feels like what im looking for are the mechanical properties of the ether?!
  11. Mar 8, 2014 #10


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    Bill_K's reply in post #2 seems intuitive enough:
    Try this: Draw a sine wave with one full frequency cycle. Now remove/erase as much of the sine wave as you want to represent a shorter pulse. When you're done erasing, draw/add vertical extension(s) to the curve that remains to connect to the zero 'node' axis. Now do you see why fractions of frequency pulse result as Bill_K described?
    Last edited: Mar 8, 2014
  12. Mar 10, 2014 #11


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