A train whistle is an everyday example of a Doppler shift
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The Doppler effect was named after Christian
Doppler, who first came up with the idea in 1842. He thought that
the frequency of sound waves would change if either the source or the
observer was moving. If they were approaching, the frequency would be
higher; if they were diverging, the frequency could be lower.
There are many everyday examples of the Doppler effect: train whistles,
police and fire sirens, race car engines. In every case, there is an
audible 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 set 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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How does the Doppler effect help weather forecasts?
