Coherence definition in physics

Coherence :

If the phase difference between two waves or phase difference between two positions of the single wave remains constant, such waves are known as coherent waves and phenomenon is called coherence.

If the phase difference between two sources changes then sources are known as incoherent. Sunlight, incandescent lamp. tube light etc is an example of the incoherent source.

There are two methods by which we can produce a coherent source :
  • Division of wavefront
  • Division of amplitude
Types of coherence :
  • Temporal coherence
  • Spatial coherence
Measurement of coherence :

The coherence of any source can be measured by the visibility of contrast of the fringe system produced by the source. It is given by 

V = Imax - Imin / Imax + Imin 

Where Imax = Maximum Intensity of bright fringe
Imin = Minimum Intensity of dark fringe