The phenomenon of non-uniform distribution of light energy in medium due to the super position of light wave at that medium at any time is called interference of light.
If crest and crest or trough and trough of two light meets at a point, The intensity of light is maximum that means bright fringes is formed and if crest and through or vice versa meet at a point, The intensity of the light is minimum that means dark fringes is formed (destructive interference).
Two sources of light is said to be coherent sources, if they emit the light wave of same wavelength (frequency), same amplitude and having zero phase or constant phase continuously. Two independent sources cannot be coherent sources. Two sources derived from the single point source is coherent sources.
Condition for interference
The conditions for interference of the light is highlighted below:-
1) Two sources should be coherent.
2) Coherent source should emit wave continuously.
3) The distance between two coherent sources should be very close to each other.