Why is it preferable to say that the doppler effect causes a shift in frequency rather than a shift in wavelength? I often read on websites that they define the doppler effect as a change in frequency.

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    $\begingroup$ Frequency is preferable as it doesn't change when the wave changes medium. $\endgroup$ – Ali Aug 27 '13 at 9:44
  • $\begingroup$ thats why as a counter example Stefan-Boltzman law and Weins law are in terms of wavelength. $\endgroup$ – user28737 Sep 27 '13 at 8:58

The doppler shift causes a shift in wavelength at the origin of the wave (the frequency of the source never changes). This results in a shift in frequency for the observer.

In the link below you can see the emission of the wave for a moving source causes the wavelength to be shorter in front and longer behind. The actual source isn't changing in frequency. So it's a matter of relativity. To the traveling observer (in the source), only the wavelength is changing, to the stationary observer (experiencing the doppler shift) both frequency and wavelength have changed.

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Lookang, Wikimedia commons. More simulations and applets here.

  • $\begingroup$ In the case of a travelling observer is there any change in observed wavelength?(when the source is stationary) $\endgroup$ – jerry Jan 26 '15 at 11:19

In case of a travelling observer , there is change in wavelength,and the magnitude and sign of change in wavelength depends on the velocity of the observer. Let's say an observer moves towards a stationary source emitting pulses with a frequency of 'f' with a velocity vo . A pulse reaches the observer and by the next time a pulse reaches the observer, the observer will have traveled a distance 'vo/f' towards the pulse. Here , relative velocity comes into picture and so the wavelength changes due to the doppler effect.


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