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Q & A: Doppler shift and wind

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Most recent answer: 12/09/2015
Supose that, in the Doppler effect for for sound , the source and receiver are at rest in some refrence frame but the transmitting medium ( air) is moving with respect to this frame. Will there be a change in wavelength , or in frequency , reveived ?
- haresh kumar (age 20)
Hyderabad, Pakistan

Very nice question. Since the distance between the sender and receiver isn't changing, there is no Doppler effect on the received sound frequency. However, the speed of transmission (in the frame of the sender and receiver) does depend on whether the transmission is upwind or downwind. Therefore the wavelength is changed. For example, say that the wind is blowing at s/2 (s is the sound speed) from A toward B. The spacing between wave-fronts traveling from A to B will be stretched out to 3/2 the length they would have if there were no wind. The spacing for the fronts traveling from B to A will be squashed to 1/2 the no-wind length. If the wind is partly at right angles to the line from A to B, then slightly more complicated effects come in. 

It was precisely the absence of such effects in the Michelson-Morley experiment for light that showed that the old picture of light as a sound-like wave in a medium which had a single "no-wind" frame could not be right.

Mike W.

(published on 12/09/2015)

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