It is the cause of the change in pitch? of a police siren as the car moves towards and away from you. The pitch is higher as the sound source approaches and lower as the sound source moves away from you.
It is important to realize that the frequency of the sounds that the source emits does not actually change. To understand what happens, consider the following analogy. Someone throws one ball every second in your direction. Assume that balls travel with constant velocity. If the thrower is stationary, you will receive one ball every second. However, if he is moving towards you, you will receive more than that because there will be less spacing between the balls. The converse is true if the person is moving away from you. So it is actually the wavelength which is affected; as a consequence, the perceived frequency is also affected.
If the moving source is emitting waves (e.g. sound waves) with an actual frequency f0, a stationary observer detects waves with a frequency f given by:
where v is the speed of the waves in the medium and vs is the speed of the source with respect to the observer (positive if moving towards the observer, negative if moving away).
The Doppler effect is not quantitatively the same depending on whether the theory of relativity is taken into account. See [relativistic Doppler effect]?.
The doppler effect has been of great use to astronomy. It has been used to measure the speed of stars moving away from us and hence the age of the universe. (See Red Shift)
The doppler effect is also used in some forms of radar to measure the velocity of detected objects.
--- ToDo? fix up: ???? Doppler first postulated the Doppler effect in ???? by standing next to a rail line and listening to a car full of muscians as they approached him and after they passed him.
Not sure if that was actually Doppler who did that. Some other physicist?
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