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I How do we know electromagnetic waves are light?

  1. Oct 23, 2018 #1
    Per the maxwell equations, we know that em waves travel at the velocity of light, but that is not a sufficient condition to say that electromagnetic waves are light. How do we know that electromagnetic waves are light? They could just be something that has the same velocity as light.

    Any insight is appreciated.
     
  2. jcsd
  3. Oct 23, 2018 #2

    russ_watters

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    I don't understand. The word "light" is just a name given to EM radiation in a particular frequency range. You seem to think they are two different things. What do you think the word "light" means?

    [edit] ...although sometimes the word "light" is used to name the entire EM spectrum. Usually context will tell you which is being used.
     
    Last edited: Oct 23, 2018
  4. Oct 23, 2018 #3

    hilbert2

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    For instance, the fact that good electrical conductors are also often mirror-like reflectors is consistent with light being an electromagnetic phenomenon.
     
  5. Oct 23, 2018 #4
    I would say that if c(the speed of light) is divided by ether the frequency or wavelength, and the result is the one you
    did not divide by, then it counts as light.
     
  6. Oct 23, 2018 #5
    What I mean is: Maxwell proved that light is an EM wave by showing that v = c via the wave function. But I don't see how showing via the wave function that v = c, automatically means that light is an EM wave.
     
  7. Oct 23, 2018 #6

    russ_watters

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    The speed of light had already been measured when Maxwell derived his equations and he merely surmised the speed being the same wasn't a coincidence.
     
  8. Oct 23, 2018 #7
    When it meets those criteria, it behaves like a wave, and works with the wave equations.
    You could say most of the same things for sound waves, but the "c" is different.
     
  9. Oct 23, 2018 #8

    George Jones

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    Yes, you asked a good question.

    I think that the experiments of Heinrich Hertz went a long way towards establishing light as electromagnetic waves. (example: above post by @hilbert2 )
     
  10. Oct 23, 2018 #9
    What?
     
  11. Oct 23, 2018 #10
    Hm. If we see light as a wave, which "wave" is the light that we usually show in diagrams, the B-field or the E-field?
     
  12. Oct 23, 2018 #11

    George Jones

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    See the attached 3-page excerpt from the 600-page book "Modern Physics" by Serway, Moses, and Moyer:
     

    Attached Files:

  13. Oct 23, 2018 #12

    ZapperZ

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    For freely-propagating wave in vacuum, it can be either one, because showing one automatically defines the other one.

    In a waveguide, this is not so obvious and the geometry of the waveguide determines what the E and B field will look like.

    Zz.
     
  14. Oct 23, 2018 #13
    C/wavelength=frequency, C/frequency=Wavelength
     
    Last edited by a moderator: Oct 24, 2018
  15. Oct 23, 2018 #14
    Thanks a lot George! That was nice of you. So now I get it: without Hertz proving that all of the other properties of EM have in fact the same results of "macroscopic" light i.e. interference, refraction, reflection polarisation and etc., then it wouldn't have been enough just to use the wave function and have shown that they both have the same velocity.

    Is this why in Optics there is first electromagnetic waves and then geometric/wave optics? Is this to essentially prove the two are one in the same?
     
  16. Oct 23, 2018 #15

    George Jones

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    Yes. For example, gravitational radiation propagates at c, but gravitational radiation is not light.

    I think it is because they have been demonstrated to be the same thing by observations of loads of phenomena, so waves are studied in optics, and the ray approximation of light also is studied.
     
  17. Oct 23, 2018 #16

    DaveC426913

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    You can blue-shift light up into UV and higher. Blue shift it enough and it will be detectable as X-rays.

    You can also red-shift it down to IR or lower. Red shift it enough and it will be detectable as radio.

    In other words, the only difference between light and other parts of the EM spectrum is the frequency.
     
  18. Oct 23, 2018 #17
    Because we have well developed instruments (cameras being one of them), which can only work given this supposition.
    Since they do work, the supposition is reasonable.
     
  19. Oct 24, 2018 #18
    I'm not totally sure we have already proved it experimentally. We have indirect indications (Mossbauer effect, galactic red shift, etc) but do we really proved it in laboratory?

    --
    lightarrow
     
  20. Oct 24, 2018 #19
    Just to add a bit to the other posts: free electron laser
    https://en.m.wikipedia.org/wiki/Free-electron_laser

    It generates light by making electrons oscillate. So light must be an electromagnetic wave.
    Of course electrons "oscillate" in a sense even when atoms emit light when they undergo a transition from an excited level to the fundamental one (for example) but I don't consider this as a real "prove" that making a charge oscillate at high frequency it generates light, we only have indirect informations that this can be considered as an actual oscillation of the charges, AFAIK.

    --
    lightarrow
     
  21. Oct 24, 2018 #20

    russ_watters

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    It has crossed into the mundane, with every day uses like weather and police radar.
     
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