What are Coherent Sources? - QS Study
QS Study

Two sources are said to be coherent if they emit light waves of the same wave length and start with same phase or have a constant phase difference.

Two independent monochromatic sources, emit waves of same wave length. But the waves are not in phase. So they are not coherent. This is because, atoms cannot emit light waves in same phase and these sources are said to be incoherent sources.