Do the two sources need to have constant phase difference or do they need to be in phase for constructive interference. I was under the impression that both sources can have constant phase difference but so long waves arrive in phase at a point, we can have constructive interference. But Sears and Zemansky seem to suggest that both sources need to be in phase.
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I can't comment on the book you're referencing, but two sources with a constant phase difference between them can cause constructive interference, so long as the fields they generate are in phase at the point where the constructive interference occurs.
answered Jul 7, 2018 at 1:04
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$\begingroup$ Just so. If the sources have a constant phase difference but are not in phase there will be an interference pattern (that is constructive, destructive and intermediate interference at different points), but the pattern will be shifted relative to the pattern that there would be if the sources were in phase. Can't believe that Sears and Zemansky would get this wrong; it must surely be some ambiguity. $\endgroup$ Commented Jul 7, 2018 at 9:29
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$\begingroup$ Don't need to have a constant phase difference for an interference pattern - it all depends on how quickly you can observe it. $\endgroup$– D DuckCommented Jul 7, 2018 at 13:52