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Radio waves travel at speed of light?

  1. Jan 18, 2007 #1
    hi all....

    Something doesn't quite add up here:

    A radio wave is an electromagentic wave, right.

    It therefore travels at the speed of light, right.

    Then why does it actually take a finite amount time (like secoonds or even miliseconds) to travel a couple of kilometers???????

    mathematically, just doesn't make sense.

    In theory, the time to travel a couple of km should be almost negligible!!
    but its not- evidently!

  2. jcsd
  3. Jan 18, 2007 #2
    1. They only travel at the speed of light in a vacuum, when propogating through a medium they are slowed according to the permeability and permativity of the medium.
    C = 1/sqrt(Permeability * Permativity)

    2. Air has nearly normal permeability and permativity so in practice they do travel a nearly the speed of light.

    What specific case are you refering to?
  4. Jan 18, 2007 #3


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    Let's see, the speed of light is just about 300,000 km/sec, so it should take 1/300,000 = 3.33 microseconds to travel 1 km. This is short but still an easily measurable time period. I've never heard of radio waves taking seconds or milliseconds to travel a kilometer or two.
  5. Jan 18, 2007 #4
    Did you take into account the time delays of the components in the emitter / receiver instruments?
    Last edited: Jan 18, 2007
  6. Jan 18, 2007 #5


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    What are you using as an example/proof of this delay?
  7. Jan 19, 2007 #6
    say for an example if i send an sms - it will only arrive at the receiveing host a second or two later.

    i suppose these time delays are due to processing the data before transmitting it???
  8. Jan 19, 2007 #7


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    That's one quarter of it, yes.
    There's also the processing at the tower, the sending to another tower, and the processing at the receiving end.

    Other than that, sure, your messages should just fly at the speed of light! :rolleyes:
  9. Jan 19, 2007 #8
    This is really funny. Did you really think that your sms's arrive with delay because of the speed of light? Then on Christmas and New Year's Eve the light must be really slow. Hey if your friend lives on an upper floor there could be some relativistic time dilation involved when you SMS him/her. :wink:
  10. Jan 20, 2007 #9


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    Not only that, but in some situations the signal might have to travel through several hundred kilometres of wiring (and switching), which does not propogate signals at light speed.
  11. Jan 20, 2007 #10
    if only i was as knowledable as you, antonantal ...

    i'm glad my desire to learn amuses you.

    to think about: “Arrogance diminishes wisdom”
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