The change in frequency of a wave observed at a detector whenever the source or detector is moving relative to one another is known as the Doppler shift. This frequency change is named after the Austrian physicist Christian Doppler who predicted its existence in 1842. The existence of the Doppler shift was first verified in experiments associated with sound waves. The Doppler shift principle has found practical application in acoustical and optical environments relating to remote-sensing, high-energy physics, astrophysics, spectroscopy, radar and meteorology.
The Doppler shift principle is used in modern acoustic flow meters to estimate the flow velocity in a pipeline. A pair of ultrasonic transmitters beam to receivers placed across the flow-line; one transmitter beams upstream and the second downstream. The detected differences in the times of travel are then related to the flow velocity.
Optical Doppler shift due to light scattered by small particles suspended in a flowing medium forms the basis of modern laser based Doppler anemometry.