Coherent sources are a pair of sources of light that emit the light waves of the same wavelength and frequency which are always in the phase with each other or have a constant phase difference. Two independent sources of light cannot be coherent sources because they may emit the lightwave of the same wavelength and frequency but they may never be in the same phase or at a constant phase difference. So, it is difficult to make coherent sources. However, two sources of light obtained from a single source can be considered as coherent sources.