The Doppler Effect

A train whistle is an everyday example of a Doppler shift Click on image for full size (36K JPEG) The Doppler effect was named after Christian Doppler, who first came up with the idea in 1842. He thought that sound waves would be pushed closer together if the source of the sound was moving toward you. He also thought that the sound waves would get further apart if the source was moving away from you. A typical example is a train. When a train is moving, it blows it's whistle. You can hear the change in pitch when a train goes by. When it is comes near you the whistle sounds higher than normal and when it passes you it sounds lower than normal. The Doppler effect doesn't just apply to sound. It works with all types of waves. This includes light. Edwin Hubble used the Doppler effect to determine that the universe is expanding. Hubble saw that light from other galaxies had a lower frequency than it should. The light he saw was red-shifted. If the other galaxies were approaching us, the light would have been blue-shifted.

How does the Doppler effect help weather forecasts?

Last modified prior to September, 2000 by the Windows Team

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