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Beats in electromagnetic waves.

  1. Sep 6, 2014 #1
    Hello all. I have been searching for information on wave beats in light but haven't been very succesful. I have found a lot of information on beats but most of it refers to sound or to waves in general without referring to EM waves.
    I think that when EM waves are dispersed at normal incidence the resultant time varying disturbance resembles beats but is different in that the different frequencies are travelling at different speeds.
    I would be grateful if anyone can refer me to some sources of information on the subject in general including methods, if there are any, to generate beats in EM waves.
    Thank you
     
  2. jcsd
  3. Sep 6, 2014 #2

    Drakkith

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    I'm not sure, but I would assume that simply interfering two EM waves of different frequencies would generate beat frequencies. I would expect this to only matter in the lower IR range and below, as above that we can't directly measure the oscillation of the electric field since it oscillates to quickly for current electronics to detect.
     
  4. Sep 7, 2014 #3
    Thanks Drakkith. I'm not yet sure either but I have the same opinion as the one you expressed in your first sentence. I'm interested in beat frequencies in general including those that may exist and in principle can be detected, but are too high to detect at the present time.
    I have been doing some more searching and have found "heterodyning". It seems to be a method, which has been around for more than a century, of creating beats in the radio region of the spectrum. I have yet to read up on it.
    I have also come across a method of creating beats by overlapping the beams from separate laser beams but I'm not yet sure about the reliability of the source of the article.
    Thanks again.
     
  5. Sep 7, 2014 #4

    Nugatory

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    Electromagnetic radiation in a vacuum is non-dispersive - the different frequencies do not travel at different speeds. And yes, the basic beat frequency equation works just fine for EM radiation in a vacuum.
     
  6. Sep 7, 2014 #5

    vanhees71

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    The detection of "Quantum beats" is a key measurement to prove the existance of photons, which is not as easy as some (mostly wrong) statements in many (otherwise good!) textbooks on quantum theory suggest, because most of what is referred to as the behavior of photons, e.g., the photoelectric effect, can as well explained by the semiclassical picture, where the electromagnetic waves are described by classical fields and only the electrons of the atoms/molecules/bulk media are described quantum mechanically. Since one was able to measure quantum beats already in 1955, I'd count that as the demonstration of beats in optics, and even more excitingly in quantum optics. There's a good description in Wikipedia (based on the very good book by Scully and Zubairy, Quantum Optics):

    https://en.wikipedia.org/wiki/Quantum_beats
     
  7. Sep 7, 2014 #6
    Thank you Nugatory. I didn't make it clear perhaps but I was referring to dispersion eg when white light in air meets a glass block at normal incidence and the dispersed rays follow the same path. I think that the resultant pattern is a bit like a beat pattern but is more involved being due to the different speeds as well as the different wavelengths.
    Thanks for confirming that beats occur for EM waves and that the equation is the same. What I find surprising is the apparent shortage of information on beats in EM waves. The main emphasis seems to be on sound. Perhaps I have been searching in the wrong places.
     
  8. Sep 7, 2014 #7
    Thank you vanhees 71. According to the article you referenced, beats in light waves can be demonstrated by the use of lasers. The scheme I referred to in post three overlaps two separate beams by using a beam splitter. I find this particularly interesting.
     
  9. Sep 7, 2014 #8
    In a plasma the beating of two light waves allows them to interact with a lower frequency wave in the plasma (electron or ion wave). If you fire a laser into a plasma these processes can scatter a lot of laser energy back out of the plasma (the processes are called stimulated Raman and stimulated Brillouin scattering in plasma physics).
     
  10. Sep 8, 2014 #9

    olivermsun

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    Radio basically works by carrying audio as "beats" of EM waves. Heterodyning is the process used to create the beat frequencies in Amplitude Modulation (AM) radio.
     
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