Phase difference between two coherent sources should be zero.

Coherent Sources:
Coherent sources are the sources of light that emit light waves of the same frequency and have a constant phase difference with respect to each other. When two coherent sources emit light waves, the waves superimpose to form an interference pattern.

Phase Difference:
The phase difference is the difference in the phase of two waves at a given point in space and time. It is measured in radians or degrees. When two coherent sources emit light waves, the phase difference between them determines the interference pattern.

Explanation:
When two coherent sources emit light waves of the same frequency and zero phase difference, the waves superimpose constructively and form a bright fringe in the interference pattern. If the phase difference between the two sources is not zero, the waves may superimpose destructively, resulting in a dark fringe.

Therefore, to obtain a clear interference pattern, the phase difference between the two sources should be zero. This can be achieved by ensuring that the two sources are of the same frequency and are in phase with each other.

Conclusion:
In conclusion, the phase difference between two coherent sources should be zero to obtain a clear interference pattern. This can be achieved by ensuring that the two sources are of the same frequency and are in phase with each other.