Interference refers to what happens when two waves pass through the same region of space a the same time. In the region where they overlap, the resultant displacement is the algebraic sum fo their separate displacements (a crest is considered positive and a trough negative. This is call the principle of Superposition. (a)Two waves have opposite displacements at the instant they pass one another, and they add to zero. The result is called destructive interference. (b)Two waves pulses overlap, they produce a resultant displacement that is greater than the displacement of either separate separate pulse. The result is called constructive interference.