In physics, the net effect of combining two or more wave trains moving on intersecting or coincident paths. Constructive interference occurs if two components have the same frequency and phase; the wave amplitudes are reinforced. Destructive interference occurs when the two waves are out of phase by one-half period (see periodic motion); if the waves are of equal amplitude, they cancel each other. Two waves moving in the same direction but having slightly different frequencies interfere constructively at regular intervals, resulting in a pulsating frequency called a beat. Two waves traveling in opposite directions but having equal frequencies interfere constructively in some places and destructively in others, resulting in a standing wave.

This entry comes from Encyclopædia Britannica Concise.
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