Two sources broadcasting identical waves create an interference pattern with bands of constructive and destructive interference. The sources above are in phase - for instance, they emit peaks at the same time.
What happens at any point depends on the path length difference ΔL, the distance from one source to the point minus the distance from the other source to the point.
Condition for constructive interference: ΔL = nλ, where n is any integer.
Condition for destructive interference: ΔL = (n + ½)λ, where n is any integer.
When the two sources are in phase you will always get constructive interference along the perpendicular bisector of the line joining the sources. This is because the path length difference for any point on the perpendicular bisector is:
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