Definitions Constructive Interference: The enhancing interference that occurs when two waves occupy the same space at the same time and both disturb the medium in the same way so that the disturbance is larger than the disturbance of either wave separately. Destructive Interference: The canceling interference that occurs when two waves occupy the same space at the same time and both disturb the medium in opposite ways so that the disturbance is smaller than the disturbance of either wave separately. Diffraction: The changing of direction of waves to bend around corners and spread as they encounter obstracles.

In the animation we show waves moving vertically upward to encounter two slits in a barrier. (You can imagine that the lines representing the waves are the crests of the waves; troughs are halfway between the crests.) When the waves encounter the slits, they diffract as they come through the slits and move upward into the space behind the barrier. Observe the pattern which is created. Where wave crests arrive simultaneously from the two holes, we create "constructive interference"; where a crest from one and a trough from the other meet simultaneously, we create "destructive interference." Observe that if one were to walk horizontally across the region of interference behind the slits, one would encounter an alternating pattern of constructive and destructive interference.

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