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. Doopler
        Effect
Click here for an interactive flash animation of the Doppler Effect.

Shock Wave

If a source of waves moves through a medium faster than waves travel through the medium, a shock wave is formed. Both the sonic boom and the wake of a boat are examples of this phenomenon.


Boat
              Wake
Sonic Boom
Boat Wake
Sonic Boom
 
Watch a movie of an F14 breaking the sound barrier.

Investigate sonic booms.

Investigate the Doppler Effect and Shock Wave Formation -- NASA Applet


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. 

Doppler Radar and Sonar

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.




Doppler Effect