The Doppler shift occurs when a source of light waves or sound waves moves towards us or away from us. If the source is moving towards us the frequency – often called pitch for sound waves - of the source appears to increase. If the source is moving away from us the frequency appears to decrease. The magnitude of the frequency or Doppler shift increases with speed, as shown below.
There are different equations for the Doppler shift for light and sound waves. The equation for sound waves is given and illustrated below.
The frequency of sound heard by the observer is
The frequency of sound heard by the observer is
Doppler Shift for Electromagnetic Radiation
(1). Here
is the speed of the observer relative to the source, negative if they approach and positive if they recede from each other. We can write an approximate expression for (1) ifis a small fraction of the speed of light.
then 
, the actual Doppler shift, given by