The Doppler effect.
![Picture](/uploads/1/8/7/8/18780660/9450998.png?1364837110)
- The change in frequency of a wave as its source moves in relation to an observer is called the doppler effect.
- The Doppler effect occurs because the motion of the source causes the waves to either get closer together or spread out, changing the pitch.
- According to the picture on the right, the waves in-front of the firetruck are moving in the same direction as the firetruck, making the waves closer together, changing the pitch.
- The waves behind the firetruck are moving away, which is also changing the pitch.
http://www.youtube.com/watch?v=Djz_rtnXSfY
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