Doppler Effect
The Doppler effect is an apparent change in the frequency of a wave,
such as sound or light, when the source of the wave and the observer
are in motion relative to one another. The frequency increases when
the source and observer are approaching each other, and decreases
when they are moving away from each other. The Doppler effect is
named for the Austrian scientist Christian Doppler (1803-1853).
An example of the Doppler effect is the change in pitch of a
train whistle
as it passes an observer. As the train approaches, the
pitch of the whistle is increased, and after
the train passes, the pitch is decreased.