When a source generating waves moves relative to an observer, or when an observer moves relative to a source, there is an apparent shift in frequency. If the distance between the observer and the source is increasing, the frequency apparently decreases, whereas the frequency apparently increases if the distance between the observer and the source is decreasing. This relationship is called Doppler Effect (or Doppler Shift) after Austrian Physicist Christian Johann Doppler (1803-1853).

The relationship describing the Doppler Shift for a moving source is given by:

where f₂ is the apparent frequency, f₁ is the actual frequency emitted by the source, v is the speed of sound in the medium, v_s is the speed of the source through the medium (the negative sign is used if the source is moving towards the observer).

The relationship describing the Doppler Shift for a moving observer is given by:

where f_o is the observed frequency, f_s is the source frequency, v is the speed of sound, v_o is the speed of the observer (it is taken to be negative if the observer is receding from the source).

The Doppler Effect explains the apparent change in pitch of a passing automobile. Of course, the frequency of the sound emitted by a source remains unchanged, and so does the velocity of the sound in the transmitting medium. A similar effect (Doppler Shift for light) can also be used to determine the speed of a star relative to the earth. The red shift of the star's spectrum indicates that the distance between an observed star and the earth may be increasing. The Doppler Shift for light describes a change in wavelength, not a change in frequency as with sound. Short range radar devices use the Doppler Shift principle. A change in frequency between emitted and returning pulses can be used to find the relative speed.