The change in frequency of a wave experienced by an observer moving relative to the source of the wave.
Can be observed when listening to police sirens.
As the car drives by, the frequency of the sound waves changes from high pitch to a lower pitch.
When waves are emitted by a stationary object, the waves spread out symetrically.
Frequency of the waves is constant.
As an object moves, it appears to catch up with the waves it is emitting.
Frequency and wavelength of the waves appears to be shortened for an observer standing in fron of an approaching object.
