The pattern produced by the interference of two coherent point sources of equal amplitude illustrates superposition in action.
The sources are coherent if they
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have the same amplitude.
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There is a constant phase relationship between the two sources.
Two loudspeakers connected to the same signal general approximate well to this ideal. They produce the pattern of sound intensity shown in the diagram below, with lines of minimum and maximum intensity radiating from halfway between the sources.
The same pattern of maximum and minimum intensity is produced by light (Young's double slit experiment) and water waves, though for light it is best observed by displaying the pattern on a screen, producing an image as below.
