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Why can't this wave travel in vacuum

  1. Dec 15, 2011 #1
    Hi Friends! Consider an Ideal condition. suppose, I prepared a device which can generate and vary the frequency of a wave (you can consider the wave to be electromagnic wave). Using this device I studied following two scenarios:
    1. I created a wave having frequency between 1014 to 1015 Hz.
    I passed this 1st waves thru vacuum, it passed easily.
    2. Then I created a wave having frequency between 20 to 20,000 Hz
    Again I passed this 2nd wave thru vacuum, but it couldn't pass.

    You must be wondering, how can I be so sure, that wave will travel or not? Answer to this is: the first wave is the frequency of visible light wave(approx), and we all know light can travel thru vacuum. The frequency of second wave is that of human audible sound range, and it cannot travel thru vacuum. Why such behaviour, when both are same wave just different in frequency?
  2. jcsd
  3. Dec 15, 2011 #2


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    These are completely different waves. There is no problem with passing a 20Hz-20kHz electromagnetic wave through the vacuum. The sound is conducted only through compressible media, so it doesn't pass through a vacuum.
  4. Dec 15, 2011 #3
    Light is electromagnetic waves, which require no medium in which to propagate.

    Sound are longitudinal pressure variations in air. They require air in which to propagate.

    Electromagnetic waves of all frequencies pass through vacuum.

    No sound of any frequency can pass through a vacuum.
  5. Dec 15, 2011 #4


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    This is where you went wron: EM radiation and sound are completely different types of waves. This question is like asking why your ears don't get wet when you hear a loud sound (since sound is like a wave on the ocean).
  6. Dec 15, 2011 #5


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    Of course, the next logical question is if you can transmit such low frequency EM waves through a vacuum, why doesn't anybody do it?

    Just take a look at submarine communications to find out why. High frequencies don't travel through salt water very well (it's an electrical conductor), so the only way to transmit radio signals to submarines is to use extremely low frequency radio signals. The lower the frequency; the longer the wavelength and it's very hard to build an antenna that can transmit such long wavelengths - not to mention your data rate is incredibly slow, as well.

    As a result, nobody transmits EM signals that low unless it's the only possible way to get a signal through.
  7. Dec 15, 2011 #6
    They are both waves, and many of the same formulas apply, like c/frequency=wavelength.
    The real difference is what c is. For EM in a vacuum it's 3 X 10^8 meters/second,
    for sound at sea level it's about 350 meters/second. The fewer the atoms the slower the speed of sound. I don't know where, but most likely around 100 torr the speed of sound drops to zero.
  8. Dec 15, 2011 #7


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    You can. But, for an object with very small dimensions compared with one wavelength of the radiation, the amount of power you can actually launch is vanishingly small. This is because the space around the object 'looks', to the object radiating the power at this very low frequency, like a very very low resistance. The internal resistance of the transmitter and antenna , although small(ish) are much higher than this radiation resistance so most of the power is dissipated within the transmitting device itself. This means that the efficiency is vanishingly low. 'good' radiators have to be a fairly big fraction of the wavelength of the signal (at least one tenth of a wavelength for anything usable)

    It is the equivalent to trying to heat up a bar of copper wire using a battery. The bar gets barely warm and the battery boils and dies.
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