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Speed of Electromagnetic Radiation

  1. Dec 9, 2008 #1
    I have a question about the speed of EM radiation.

    As far as i understand, all Frequencies of EM radiation travels at the same speed. ie, will reach point A to point B in the same amount of time.... But wouldnt that mean that as you go from one end of the spectrum to the other, the waves are travelling at different speeds due to the fact that the wave length and amplitude are changing?

    Kind of like a straight line, 10 mile care race between two cars in which they have to zig zag the entire race. One car has to zig zag 50 times and the other car has to zig zag 100 times. (50hz and 100hz) Each cars zig zag pattern travels the same distance to the left and right of the line. (equal amplitude) now if both cars complete the 10 miles in exactly the same amount of time then technically one car actually travelled faster because he drove a longer distance. Due to zig zagging 50 more times than the other.

    Would this not be true for different frequencies of the EM spectrum? or is our "up and down" "back and forth" wave concept really just for our own mental picturing?

    Thanks anyone.
  2. jcsd
  3. Dec 9, 2008 #2
    As per my question above....

    Im not a physicist, just a dude. so sorry if i offend anyone with my noobiness. :)
  4. Dec 9, 2008 #3
    Your analogy with the cars zig-zagging is not how EM waves work.

    EM radiation behaves like a wave in certain aspects, but it is not for example similar to a wave on a rope, or a wave on the water.

    If you look at the EM-wave as actually something that is waving, then a point on a wave of 100 Hz would move twice as fast ('up' and 'down') as a point on a wave of 50 Hz. However, that is not what is meant by the speed of EM radiation. (Also, there is nothing waving in EM radiation.)

    The speed of EM radiation in a vacuum is always equal to c, the speed of light (because light is also EM radiation), no matter what its frequency is. A higher frequency simply means a shorter wavelength, so if you would look at the 'wave' you would see more peaks and troughs, than a wave with a lower frequency.

    I am not sure, but if you have EM radiation in some material, I think the speed could be frequency-dependent.

    So yes, I guess you could say the 'up down back and forth' wave concept is really just for our mental imaging. Again, nothing is moving in an EM-wave. EM radiation can travel through a vacuum where there is nothing to wave, unlike for example sound waves, which need a medium (air for example) to propagate.
  5. Dec 9, 2008 #4
    Thanks for the quick response.

    So then why do we use the wave analogy? Are the peaks and troughs then somehow related to negative and positive charges? Like how we end up with a sine wave when graphing the voltage changes in AC electricity?
  6. Dec 9, 2008 #5
    An EM wave, as the name implies, consists of an electric field and a magnetic field.

    The strength of these fields vary in time, usually like a (co)sine wave. While there is nothing moving or waving, the strength of the EM field varies in time.

    See also http://en.wikipedia.org/wiki/Electromagnetic_radiation for a general description.
  7. Dec 9, 2008 #6
    Ahhhh. Cool that makes sense. I was always looking at a graphed out waveform as actual physical movement from one location to another. I see how i was oh so wrong.

    Thanks again.

  8. Dec 9, 2008 #7


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    Staff: Mentor

    I'm not sure that this was made sufficiently clear (though I mostly just skimmed the thread): when talking about the speed of any wave, it is the linear speed of the whole wave that is being described, not the speed of the individual particles. Note that in a propagating wave, particles do not have a net motion: when the wave passes, they are where they started before the wave passed. For waves on the ocean, a bobbing buoy (or a water molecule on the surface) will move in a circular pattern as the wave passes. For sound waves, an air molecule (or the drapes next to your subwoofer) will move back and forth.
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