two sources of waves are said to be coherent if a definite phase relation ship exists between the waves.
coherent radiation is obtained by Laser action
Incoherent sources have no changing phase difference between them.
The Doppler effect, is the change in frequency and wavelength of a wave for an observer moving relative to the source of the waves. For waves that propagate in a medium, such as sound waves, the velocity of the observer and of the source are relative to the medium in which the waves are transmitted. The total Doppler effect may therefore result from motion of the source, motion of the observer, or motion of the medium. Each of these effects is analysed separately.