The phenomenon that causes the pitch of a passing train's whistle to sound higher than usual as it approaches, and lower than usual as it travels away, is called ___.

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The Doppler effect is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. It is commonly observed in sound waves, such as the change in pitch of a passing train's whistle as it approaches and then passes by.

The Phenomenon That Causes the Pitch of a Passing Train's