8.3.3—The conditions required if two-source interference fringes are to be

To see steady two-source fringes, sources should have the same frequency and a constant phase relationship; the geometry must also allow path differences to vary.

Similar amplitudes make maxima and minima distinct, and a screen far enough away can make fringe spacing easier to resolve.

Independent lamps usually wash out visible fringes because their phase difference changes randomly, whereas one laser split into two paths can remain coherent.

Equal source distance alone does not establish coherence; coherence is about phase stability over the observation time.