For two sources to produce a steady, observable interference pattern they must be
- A. of different frequencies
- B. coherent, keeping a constant phase relationship
- C. very far apart
- D. of unequal amplitude
Explanation
If the phase difference between the sources drifts, the positions of the bright and dark fringes shift too fast to be seen and the pattern washes out. Coherence in practice means the same frequency and a fixed phase relationship, which is why Young used a single source split into two rather than two separate lamps. Equal amplitudes are not required, but they give the greatest contrast.
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