via Roeland Otten.
Imagine first some ripples on a pond. Fig 1 If I start my ripples by disturbing the water at just one point near one side, they would spread as out as semi circles. If I make more waves starting from a place close by to the first source, the waves will spread out from this second point and as the waves cross over each other they make a characteristic pattern called interference. It is a misleading name as they interact in a very orderly way. Visual artists familiar with op art may remember something called the moiré pattern. If you overlay two parallels sets of lines, or concentric arcs of circles in this case, the lines produce strong fringes where they overlay each other. The waves do the same thing: where two peaks of a wave meet they reinforce, where a peak meets a trough they cancel out. If you do an experiment with light waves which are microscopically small and pass them through two narrow slits, interference patterns form and are visible to the naked eye since they can be much larger than the waves that produced them. It takes a little care to set the experiment up so that the pattern is large enough to see but it is fairly easy using a laser. If we put a screen behind the double slit for the light to fall on, then we would see a series of fringes as shown in fig 2.
Automated Doors, 2013