The phenomenon by which the observed frequency of a wave changes as a result of a time change in the effective length of the path of propagation between the source of the wave and the point of observation. If there is a source of wave energy and an observer of the wave energy, the frequency of the waveform increases as the observer moves closer to the source, the source moves closer to the observer, or both.The frequency of the waveform decreases as the observer and source move farther apart. The phenomenon applies to all waveforms, including acoustical and electromagnetic waveforms. In acoustics, the pitch of the sound increases as the observer and source move closer together, and decreases as they move farther apart, as you may have noticed when listening to a train whistle as the train comes closer and then goes farther away. The combination of the Doppler effect and that of multipath fading causes the wooooo-wooooo sound. In telecommunications, the Doppler effect creates difficulties when a mobile device, such as a cellular telephone moves towards or away from a fixed base station at a high rate of speed.The Doppler effect is used in some forms of radar to determine the speed and direction of a moving object.The Doppler effect was first hypothesized by Johann Christian Andreas Doppler (1803