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Help Layman Understand EM Wave Propagation

  1. Apr 10, 2010 #1
    Hello everyone. I have tried to do as much research as my layman mind will allow on how an electromagnetic wave propagates in relation to how a sound wave for example does.

    I understand that an acoustic wave is longitudinal and works on compression and that a light wave is a transverse wave that propagates perpendicular to the direction.

    But I just don't understand how exactly it is propagating. I mean I know that science probably doesn't 100% understand it either but can someone clear up any of it at all? Or is it a complete 100% mystery?

    For example let's say light is a packet of photons and they are traveling in a straight line, what is it exactly that is acting as a wave? If it is the photons that are transversely waving from side to side, what is it that's making them go from one side then come back to the other instead of flitting away in random directions? Is it the 'magnetic field' property? Is there basically a magnetic field around each photon that is making it swing like a pendulum in frequencies?

    How does it work.......?

    Thank you.
  2. jcsd
  3. Apr 11, 2010 #2
    From a classical electromagnetic theory, light can be thought of as a transverse wave in the electromagnetic field of a charged particle. So if I have a charged sphere and wiggle it, it will give off an electromagnetic wave that travels through the EM field of the sphere. Some phenomena can only be understood using this wave idea. Similarly, some phenomena can only be understood by thinking of light as a discrete particle like entity. The view of quantum mechanics is that the phenomena are real, but the wave/particle pictures are just stories we tell ourselves about what is happening. When you picture a photon as a little ball or point travelling in a straight line from point A to point B, that isn't a good way to think about it. Picturing it as a ball may sometimes help to explain what happens at B when its detected, but we'd be in error if we assumed it was a ball that travelled in straight lines all the time. The main thing is that if we accept both pictures, we can describe all the phenomena...they are opposite and contradictory, but they compliment each other. As Bohr would say, opposites are compliments.
  4. Apr 11, 2010 #3

    Claude Bile

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    Photons are themselves quanta of an EM wave, they are not the medium in which the "vibration" is occuring.

    Additionally, EM waves do not represent oscillations in displacement (like water waves, or sound waves do), but oscillations in the amplitude of an electromagnetic field.

  5. Apr 11, 2010 #4
    EDIT: Claude Bile beat me to it. I didn't see his post, sorry.

    Our normal intuition, built up from everyday life, leads us to believe that all waves must propagate in some medium. This idea is false for light, as I'm sure you probably know.

    An example of transverse waves would be ripples in a pond. However, you should not take this analogy too far as water waves occur on a 2-dimensional boundary while light waves occur in 3-d space. Also, water waves have only one component (height) while EM waves have electric and magnetic components. Ultimately, we are incapable of visualizing EM waves directly. The best you can do is mathematical manipulation.

    Light is not a complete mystery. In fact it is quite the opposite; QED explains light so perfectly that you could say light is probably one of the best-understood phenomena in the Universe. Now gravity, that is what I would call tricky.

    Photons do not wave around, at least not in the classical sense of waving around. The DeBroglie picture is not wrong though, just incomplete.
    In a sense, the photons are the waves.
    Again, both the wave picture and the particle picture of light are both analogies and abstractions of a deeper physical truth, part of which has been captured in QED. You should not take these analogies too far as they will lead you into trouble. You should also know that these problems are resolved with the mathematics of QED.
    Last edited: Apr 11, 2010
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