Describes the change in a wave's frequency experienced by an observer moving in relation to the wave's source. In the case that the wave source and observer are moving toward one another, the frequency of the observed waves increases and wavelength decreases. With sound, this would result in an increase in pitch. Conversely, for a wave source and observer moving away from one another, the wave frequency decreases and wavelength increases. With sound, this would result in a decrease in pitch. The Doppler Effect can be experienced with any wave form, including light (see redshift and blueshift).