The Doppler effect was first proposed in 1842 by an Austrian physicist Christian Johann Doppler. Consider a car emitting sound waves in all directions having constant velocity, frequency, and wavelength. The shape of the wavefronts coming from the source is described by a series of concentric, evenly spaced circles. An observer standing on any side of the stationary source will encounter each wavefront with the same frequency that is emitted by the source. Now, consider a fire truck passing by a stationary observer. In this case, the man observes both the high and low pitch of the sound waves. The change in the observed frequency of the sound waves, when there is relative motion between the source and the observer, is known as the Doppler effect.