According to wave optics, light is a form of energy. The source of light sends out disturbance in all the directions. The particles which are at the same distance from the source reach at the same time. These particles are said to be in phase. Example – Spherical wave front; S is the source; All particles on the sphere at a distance r from the centre are in phase with each other. Hence, the phase difference between them is zero.

Consider incoherent sources where two sources of light which do not emit light waves with constant phase difference. Superposition of such light waves from such sources does not show up interference.