A train whistle is an everyday example of a Doppler shift.
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The Doppler Effect
The Doppler effect was named after Christian Doppler, who first came up with the idea in 1842. He learned that sound waves would be pushed closer together if the source of the sound was moving toward you. He also learned that the sound waves would get further apart if the source was moving away from you.
An example of the Doppler effect is a train. When a train is moving and blows it's whistle, you can hear the change in pitch when the 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.
Doppler radars also help meteorologists learn about possible tornadoes.
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