interference pattern

An interference pattern is the alternating arrangement of bright and dark regions that forms when two or more waves overlap and combine. Where the crests of the waves line up, they reinforce each other and the result is intense — constructive interference, the bright bands. Where a crest meets a trough, they cancel and the result is dark — destructive interference. The regular striping is the fingerprint that something genuinely wave-like is at work.

What decides whether waves add or cancel at a given spot is the difference in how far each wave has travelled to get there, which translates into a difference in phase. A path difference of a whole number of wavelengths brings the waves into step and makes a bright fringe; a difference of half a wavelength puts them out of step and makes a dark one. The spacing of the fringes therefore encodes the wavelength of whatever is interfering.

In quantum mechanics the same arithmetic governs particles, but now it is probability amplitudes that add and cancel. The bright fringes are places where a particle is likely to be detected, the dark fringes places where it almost never is. The pattern is not painted by many particles bumping into one another; it is the shape of the single-particle probability distribution, revealed only after many independent detections are tallied up.