The Doppler Shift and Its Practical Applications

Listen to the sound of a train whistle as it approaches then passes you and you will hear its pitch change from a high note to a lower one as it goes by you. Pitch is directly related to frequency-the higher the frequency, the higher the pitch-the lower the frequency, the lower the pitch. It is only at the moment you and the train are next to each other that you hear the true pitch of the horn-usually so fleetingly, you won't notice it. This is the most commonly known demonstration of the Doppler Shift or Doppler effect.

What is the Doppler Effect? What Causes It?

The Doppler Shift or Doppler effect is named for Austrian physicist Christian Doppler (1803-1853), and represents the apparent change in frequency of a medium being emitted from a moving object. Thus the Doppler effect requires at least two velocities-that of the object itself, and that of the medium it is emitting. The observer may or may not be moving. If an object is moving toward the observer, the velocities are additive. If the object is receding, the velocities are now subtractive (although the object is receding, the emitted medium is not). Put another way, in the approach, the velocity is greater than that of the radiation alone, and in the recession, the velocity is less than that of the radiation alone.

I hear the Train a-Comin'

In the case of the railroad train above, the individual at the intersection is stationary. The two velocities are that of the train and that of the sound wave emitted by the train's horn. As the train approaches the listener, the sound heard (head on) has a total velocity equal to that of the sound wave added to the velocity of the train. To visualize this, see the sound wave demos on the Kettering University associated webpage.

When the train has passed, the velocity of the note heard by the listener (if directly behind the train) is equal to the velocity of the sound wave from the horn minus the velocity of the train-or, more accurately, the train velocity is now negative and is added to the positive velocity of the sound. Thus, the Doppler effect is the shifting in sound or perceived frequency that occurs as the train approaches, reaches, then recedes from an observer. The Doppler effect is not limited to audio signals, but applies as well to other electromagnetic frequencies such as visible light. A third velocity, that of a moving observer, can be added into the equation. On rare occasions that the medium is moving, that velocity, too, is figured in.

Applying the Doppler Shift to Astronomy

In the field of astronomy, the velocity of light radiation replaces that of sound. The relative rate of recession, or of approach of space objects produces the Doppler shift. If the object is going away from the observer, the shift is "red," representing an apparent decrease in frequency of the light. If it is approaching, the shift is "blue," representing an apparent increase in light frequency. Where astronomical shifts are studied, new insights reveal it is important to recall that there can be other factors involved that complicate evaluation.¹ This becomes transparently obvious when redshifts indicate an object is traveling at a velocity greater than the speed of light.

Applications - in Space and on Earth

By viewing a moving object from more than one observation point, the change in frequency, whether red or blue, and how much of a change there is, indicates the speed of a moving object. Since many of the shifts are red, in the past, this has been used to suggest the universe is expanding. In a more localized and perhaps more useful way, the Doppler effect can be used to determine the motions of individual objects within a galaxy, etc.

Here, on Terra firma, the Doppler shift is used in police and other radar systems. A sound is bounced off a moving object (for instance, an automobile) and the apparent change in frequency is detected, indicating it is or is not breaking the speed limit. Also, radar can be used to track the motions of storms-this is best known as Doppler radar. Finally, the Doppler effect can be used to track the motion velocities and directions of blood within the heart, giving us the echocardiogram.

¹ Astronomy Cafe - Galaxy Redshifts Reconsidered

References and Resources:

National Center for Supercomputing Applications, University of Illinois - The Doppler Effect

Kettering University, Personal Accessible Web Space - "The Doppler Effect and Sonic Booms," by Daniel A. Russell

How Stuff Works - Practical Applications of the Doppler Effect