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Related to coherent sources: phase difference
If light beams from two independent sources reach the same point in space, there is no fixed relationship between the phases of the two light beams and they will not combine to form interference effects. Such light waves are called incoherent. If, on the other hand, the two light beams are superimposed after reaching the same point by different paths but are both radiated from one point of a source, interference effects will be seen because the phase difference in the two beams is constant. The two virtual sources from which these two beams are apparently coming are called coherent sources and any rays in which there is a constant phase difference are called coherent rays. Prior to the advent of the laser, the only way in which one could obtain coherent rays was by dividing the light coming from a point source into two parts. See Young's experiment; holography; clinical maxwellian view system; optical coherence tomography.