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What do Electromagnetic waves actually look like?

  1. Sep 10, 2012 #1
    Ive asked this question so many times on various forums and i feel there is a simple answer that i just havent heard yet. I realize that an EM wave is represented as a sine wave, but from what i understand, thats not the physical motion the fields take. Lets say we take a snapshot at the peak of the wave. Now what does that physically consist of. Does the electric field just stick straight up in 1 dimension, a straight line, and the magnetic field the same but parallel? I just cant picture it
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  3. Sep 10, 2012 #2

    Simon Bridge

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    The field does not look like anything.

    It is not a field of anything - an EM wave is not a wave in some medium like water waves. The word "field" being used here is a technical term in mathematics.

    The classical EM field is usually thought of as a description of the size of potential effects in space ... defined in terms of the motion of charges.
    So this may help you: if water were invisible - you could only see the waves by the stuff floating on it. The motion of the floating stuff can be quite complicated - but you can isolate components by constraining the motion to one dimension: eg. a cork bobbing in a tube.

    It is just like that with EM fields/waves - the charged particles do the floating ... only there's no water.

    The physical object associated with a field is the photon - they "look" like sharp hits (eg. they are, physically, particles - wave effects get built out of the statistics of many of them). To work with their fields properly you need Field Theory.

    Nobody can picture it except through the mathematical representations. You just have to live with it: these are the rules that our Universe works by.
    Last edited: Sep 10, 2012
  4. Sep 10, 2012 #3
    Wow that is a cool way to explain it. Do you know of any animations or models that show how a 3d array of charged particles would react to an EM wave?
  5. Sep 10, 2012 #4
    EM is a field excitation. It really has nothing to do with particles or waves. We use these concepts because they are familiar to us and because the field excitations sort of kind of act like particles and waves, but eventually you get to a point where these crutches get in the way. It doesn't help that there is no agreement as to any kind of visualizable model. As of today you just have to pay close attention to the main experiments -- Stern-Gerlach, Double slit, interferometers with one quantum, special relativity, and EPR/Bell -- and come up with your own concept. Just keep in mind that when physicists speak of particles and waves they really mean sorta-kinda-particles and sorta-kinda-waves. EPR/Bell is particularly weird. In my opinion it is so mind-boggling that most physicists just give up and use the math. Some of them deliberate avoid having an intuitive concept because it always comes out wrong and does more harm than good.

    That all being said, I'll work with the kinda-sorta-wave concept. You have to start somewhere. So check out the attached file. The wave is a helix in a higher dimensional space. The electric field aspect is a projection into our space from one direction, and the magnetic field aspect is a projection into our space from another direction 90 degrees away.
    As you can see there is a circle that exists in these two imaginary dimensions that generates this helix by rotating as it moves. I don't think that that really happens, but it is good enough of a model for now.

    Usually this helix is superposed with a helix that is rotating in the opposite direction. The two helices may or may not be synchronized. The synchronized thing, just think of two circles rotating in opposite directions. The unsynchronized thing I'm not going to mess with. Let's leave that as an exercise for the reader. :-)
  6. Sep 10, 2012 #5


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  7. Sep 11, 2012 #6

    Simon Bridge

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    I got it off Einstein ...
    ... he get's everywhere I tell ya.

    I don't think there are any animations of the kind you are thinking of ... the equivalent to a cork in a tube bobbing on water waves would be electrons in an antenna. They just go back and forth with the EM field.
  8. Sep 11, 2012 #7


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  9. Sep 11, 2012 #8
    So is there an actual physical quantity to the amount of photons experienced? Say i just see a duration of a 1 photon thick wall of photons hitting my eye in all directions that my stationary eyes can see. How close together are these photons that make up a complete image to my eye. Does that make sense haha? Its kinda hard to explain but im not seeing tiny gaps of blackness when i look somewhere, it seems that the photons are packed extremely tight together.
  10. Sep 11, 2012 #9

    Simon Bridge

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    It's called "brightness" or "intensity" of the light, when you are talking ab out the experience via the visual system. The number of photons per second through some area is called the flux.
    No. Not really.
    Your visual system is very good at papering over the gaps to make your visual experience appear seamless ... it is nothing like, say, a movie which presents images too fast to see the gaps either (but old movies flicker) and there is substantial processing while you see something happen and after the fact in your memory.

    You will be familiar with the blind-spot where the optic nerve goes through the retina? You don't normally see it right? But it is better than that: for instance, you experience color vision right to the edge of your eyesight - yet the color-processing parts are only in the middle - iirc the part of your visual field actually getting color information is the size of your thumbnail held at arms length. What happens is you brain recalls what color everything is and colorizes the picture to the edges.

    Neuroscientists have a lot of fun exploring this sort of thing.

    Dan Dennet does a good job of summarizing the different oddities of the human sensory system in Consciousness Explained.
    Last edited: Sep 11, 2012
  11. Sep 12, 2012 #10
    Can't access that but had recognized it - despite your difference in interpretation. :tongue2:

    "Think of waves on the surface of water. Here we can describe two entirely different things. Either we may observe how the undulatory surface forming the boundary between water and air alters in the course of time; or else — with the help of small floats, for instance — we can observe how the position of the separate particles of water alters in the course of time. If the existence of such floats for tracking the motion of the particles of a fluid were a fundamental impossibility in physics — if, in fact, nothing else whatever were observable than the shape of the space occupied by the water as it varies in time, we should have no ground for the assumption that water consists of movable particles. But all the same we could characterise it as a medium."

    - http://en.wikisource.org/wiki/Ether_and_the_Theory_of_Relativity
  12. Sep 12, 2012 #11
    I read somewhere (don't know where) that picturing a literal spherical transverse wave is impossible (and indeed I tried in vain). If so, then already for this simple reason the simple transverse spherical wave model can't be correct.

    What can be pictured (as a model!) is along one direction (a light ray) or eventually two directions - see the links that others gave and also this one:
    Last edited: Sep 12, 2012
  13. Sep 12, 2012 #12

    I used to mess with radio and realized how mysterious it was. Wiggle electrons over here in a device with a certain geometry and other electrons in a device with a similar geometry wiggle over there. Who knows why they do it, they just do. We've got math that predicts what will happen, that's all.
  14. Sep 12, 2012 #13

    The waves in that graphic are in phase. Shouldn't they be 90 degrees out of phase? I attached a better graphic to my post, but that seems to have been futile as I can find no way to access attachments! :-)>
  15. Sep 12, 2012 #14
    Good one! There has been at least one discussion about that issue, see:
  16. Sep 12, 2012 #15


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    The free space impedance (and that of a resonant antenna) is resistive not reactive so they should be in phase.
  17. Sep 12, 2012 #16

    Simon Bridge

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    Nice find.
    Notice how careful he is about the idea that the ether does not exist? At the time it was the established theory with experimental support. Overturning it needs attention to detail.
    Science does not do "why" questions. Anyway, the same can be said of "contact" forces.
  18. Sep 12, 2012 #17


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    That sounds like an excellent answer and bomb proof.
    If you take other forms of wave (sound, for instance) and consider the way that the pressure (potential energy) and the 'flow' (kinetic) vary, they are in quadrature. I find that confusing if you associate E with potential and H with current (kinetic). I guess that's a wrong analogy.
  19. Sep 14, 2012 #18
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