Why can’t two independent sources of light be coherent?
Two independent sources of light cannot be coherent because they do not maintain a constant phase difference with time.
To get the interference of light light should have coherent.
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Explanation
Each independent source of light emits light waves due to the random and independent transitions of its atoms.( Even a small sources of light emits millions of atoms)These atomic transitions occur randomly, so the phase of the emitted waves changes irregularly. Therefore, the phase difference between the waves emitted by two independent sources changes continuously with time .Hence, they cannot produce a sustained and stable interference pattern.
(Conclusion of this explanation explain that “Two sources are coherent only when they emit light waves of the same frequency and maintain a constant phase difference”.)
Example: Two separate electric bulbs are independent sources and cannot produce a stable interference pattern.
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