what coherence really mean in the sence of waves ?
Technically coherence may be defined as wave property for which corresponding points on the wave front are in phase. For our purpose however we may look upon it as describing a wave of a single frequency. This is very different from the incoherent light always around us, for example sunlight is a mixture of many frequencies and most artificial lighting is similar.
Coherence can be classed in two ways: spatial and temporal coherence. Spatial coherence refers to all the light along the surface of an emitter being in phase: imagine a solid sphere where all the light leaving every part of the surface is perfectly in phase. Temporal coherence pertains to all the waves being perfectly in step at all times so that, if the phase is measured at some time t = 0, and is found to be identical for all the emitted light, the light beam can be said to be temporarily coherent.