Definition of Coherent Sources
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