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

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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.

enter image description here

Lookang, Wikimedia commons. More simulations and applets here.

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