Fri February 15, 2013 By: Aditya Apte
The Wave optics Study material has the following statement for  Coherent sources :- "Two sources are 
coherent if they have the same frequency and a stable phase difference."  

Definition of Coherent Sources

Expert Reply
Mon February 18, 2013

Two light sources are said to be coherent if the waves emitted from them have same phase and same frequency.

For example:- Laser light produce coherence.

Coherent source of light are those source of light which emit light wave of same wavelength, same frequency and in same phase or having the constant phase difference.

Two independent sources can never have all these factors same at a time. Hence, two independent sources of light can 't be used as a coherent sources

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