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Doppler Effect

  1. Feb 26, 2006 #1
    So in the case where the wave source is moving towards a stationary observer, the observer will observe a frequency f' where f'=f(v/(v-vs))

    f= frequency of wave
    v=velocity of wave
    vs=velocity of source moving towards observer

    I understand that the increase in observed frequency is caused because the wavelength of the wave is shortened since the source is moving toward the observer. In other words, more waves can be crowded into the area between the source and observer than if the the source was at rest.

    My question is this, why does the velocity of the waves emitted not change when the source is moving? Why does the observed frequency increase as a result of decreased wavelength and not as a result of increased velocity?

    Please tell me if my question is not clear so I can rephrase.
  2. jcsd
  3. Feb 26, 2006 #2


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    The short answer to your question is that speed of the wave is function of the properties of the medium and not the source.
  4. Feb 26, 2006 #3
    The way I think of it is that once the sound waves are emitted and leave the source, they are completely independent of their source and have their own constant speed that they would otherwise have no matter what the source is doing. Then when the source moves toward that wave, it catches up to it somewhat and emitts another wave which is also that same constant speed but is now a little closer to the previous ones so it's artificially like a different frequency but all those waves are still at the same constant speed.
  5. Feb 26, 2006 #4
    The velocity of a wave is only determined by the median in which it travels.
    Sound in air for example: The tiny particles of air take time to each slam into each other transfering their energy to the next(a compressional wave) to travel a distance. It doesn't matter how fast whatever object was that made the disturbance in the air. The particles of the median will only bounce into the next particle at a certain speed. It's kind of like the reaction time of the median(in this case the air).

    The wavelength gets smaller when you make a disturbance really close to the next one. When you are moving with a velocity and say you clap your hands, the sound wave will travel the speed of sound. When you clap your hands a second time, the second sound wave will be closer than the last one because you were moving closer than the speed of sound than if you were just standing still or going in the opposite direction, thus a higher frequency.
  6. Feb 26, 2006 #5
    Thanks for your replies. This is clearing things up.
  7. Feb 26, 2006 #6
    Thanks for your replies. This is clearing things up.
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