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 have a higher frequency if the source was moving toward the observer and a lower freqency if the source was moving away from the observer.
A commonly used example of the Doppler effect is a train. When a train is approaching, the whistle has a higher pitch than normal. You can hear the change in pitch as the train passes. The same is true with sirens on police cars and the engines of race cars.
One way to visualize the Doppler effect is to think of sound waves as pulses emitted at regular intervals. Imagine that each time you take a step, you emit a pulse. Each pulse in front of you would be a step closer than if you were standing still and each pulse behind you would be a step further apart. In other words, the frequency of the pulses in front of you is higher than normal and the frequency of the pulses behind you is lower than normal.
The Doppler effect doesn't just apply to sound. It works with all types of waves, which includes light. Edwin Hubble used the Doppler effect to determine that the universe is expanding. Hubble found that the light from distant galaxies was shifted toward lower frequencies, to the red end of the spectrum. This is known as a red Doppler shift, or a red-shift. If the galaxies were moving toward Hubble, the light would have been blue-shifted.
Doppler radars also help meteorologists learn about possible tornadoes.
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