Doppler Effect Statement:
The Doppler effect, named after Austrian physicist J. C. Doppler who first described it for sound in 1842, states that “The waves emitted from a source moving toward an observer are squeezed; i.e. The wave’s wavelength is decreased and frequency is increased,” as shown in the schematic below. Conversely, waves emitted from a source moving away from an observer are stretched; i.e. the wave’s wavelength is increased and frequency is decreased. The waves can be acoustic waves or electro-magnetic radiation.
Laser Doppler Velocimeter:
The laser Doppler velocimeter sends a monochromatic laser beam toward the target and collects the reflected radiation. According to the Doppler effect, the change in wavelength of the reflected radiation is a function of the targeted object’s relative velocity. Thus, the velocity of the object can be obtained by measuring the change in wavelength of the reflected laser light, which is done by forming an interference fringe pattern (i.e. superimpose the original and reflected signals).