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 determined that the frequency of sound waves would change if either the source of the sound or the observer was moving. If they were approaching, the frequency would be higher. If they were diverging, the frequency would be lower.

There are many everyday examples of the Doppler effect: train whistles, police and fire sirens, and race car engines. In every case, there is an noticeable change in pitch as the source approaches and then passes an observer.

One way to visualize the Doppler effect is to think of sound waves as pulses emitted at regular intervals. Imagine walking forward. Each time you take a step, you emit a pulse. Each pulse in front of you would be be a step closer together than if you were emitting them while standing still; each pulse behind you would be a step further apart. The pulses in front of you have a higher frequency than at rest, and the pulses behind you have a lower frequency at rest.

The Doppler effect doesn't just apply to sound. It works with all types of waves, including light. Edwin Hubble used the Doppler effect to determine that the universe is expanding. Hubble found that the light emitted by the galaxies was shifted toward lower frequencies, or the red end of the spectrum. This is know as a red Doppler-shift, or a red-shift. If the galaxies were moving toward Hubble, the light he observed would have been blue-shifted.

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