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Change in Frequency/Wavelength in Doppler Effect

  1. May 29, 2015 #1
    Greetings ,
    Many Doppler effect questions require to find the final Frequency/Wavelength. so after using the equation :
    Δλ / λ = v / c or Δf / f = v / c
    How can I know if the change in Frequency/Wavelength is positive or negative to calculate the final frequency or wavelength. because sometimes they add the change to initial but in others they subtract .

    i know this's all related to :
    * Δf= ff - fi or Δλ=λf - λi
    * Source approaching: frequency increase , hence wavelength gets shorter
    * Source moving away : frequency decrease , hence wavelength gets longer
  2. jcsd
  3. May 29, 2015 #2
    One way to imagine this fact is to thinking about the distance. If distance get shorter then λ get shorter. Source frequency does not change but for the adapter it likes to be larger.
  4. May 29, 2015 #3
    It will depend on the situation. You need to understand various cases (there are 4 cases for sound) and pick-up the right sign.
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