DOPPLER EFFECT
You may have heard the changed
pitch of a train whistle or a car horn as the train or car
approached and then receded from you. This change in pitch results from
the relative motion of the source of the sound (train or car) and the
observer (you). When the train is moving toward you, the wave crests
are closer together and the whistle's pitch sounds higher. When the
train is moving away from you, the wave crests are farther apart and
the whistle's pitch sounds lower. This is illustrated in the drawing by
Paul Hewitt shown below.
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| Boat Wake |
Sonic Boom |
Red Shift
Visible light along with the other members of the electromagnetic spectrum, which travel at 300,000,000 m/s, exhibit a Doppler shift resulting from relative motion of the source and observer. Astronomers have utilized this property to deduce that the universe is expanding.
When a wave is reflected from a moving target, the reflected wave is Doppler shifted as if the wave were being emitted from the target. When a signal (either radar or sonar) of known frequency is reflected from a moving target, the frequency of the reflected wave can be used to determine the velocity of the target. Radar waves are used in conjunction with this principle by police to monitor vehicle speeds and by meteorologists to track storms. Sonar (sound or ultrasound) is used in an analogous manner to observe vessels, fish, and ocean currents.