Christian Doppler was a scientist and mathematician who explored and explained the phenomenon that has come to be called the Doppler Effect. He noticed that sounds seemed to be of higher pitch when the listener and the source of the sound were approaching each other, and of lower pitch when they were moving away from each other.

To verify this he had a musician on a train play a note while someone stood to the side listening and trying to determine what note was being played. As the train approached, the listener thought the note was higher than what the musician was playing. As the train passed, the listener heard the pitch drop, and as the train moved away the note sounded lower than what the musician was playing and hearing.

Click on the image above for a simulation.

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