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Homework Help: Doppler Effect (Proving equal wavelengths )

  1. Sep 21, 2011 #1

    I am doing doppler effect and this question wants me to prove that the wavelength perceived by the observer and that of the wave emitted is the same.

    I tried doing it but couldn't, so I checked out what they had as an answer:

    They said that the wavelength of the emitted wave is: v / f, which I agree with

    They said the perceived wavelength is equal to: u + v / f (1 + u/v), which I also agree with.

    Now, how they proved that those are equal I don't understand... I tried but I just can't see it. Can anyone help me with the steps involved?

  2. jcsd
  3. Sep 21, 2011 #2


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    Homework Helper

    it is just transformation of the expression, using common denominator.

    (1 + u/v) = (v/v + u/v) = (v + u)/v = (u + v)/v [I reversed the v+u so it was identical to the numerator]

    Put that in the expression and the (u + v) terms cancel, and the v ends up in the numerator to give v/f
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