15.1. Double Slit Interference http://www.ck12.org
Any two waves in the same medium undergowave interferenceas they pass each other. At the location where the
two waves collide, the result is essentially a summation of the two waves. In some places, a wave crest from one
source will overlap a wave crest from the other source. Since both waves are lifting the medium, the combined wave
crest will be twice as high as the original crests. Nearby, a wave trough will overlap another wave trough and the new
trough will be twice as deep as the original. This is called constructive interferencebecause the resultant wave is
larger than the original waves. Within the interference pattern, the amplitude will be twice the original amplitude.
Once the waves pass through each other and are alone again, their amplitudes return to their original values.
In other parts of the wave pattern, crests from one wave will overlap troughs from another wave. When the two
waves have the same amplitude, this interaction causes them to cancel each other out. Instead of a crest or a trough,
there is nothing. When this cancellation occurs, it is called destructive interference.
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It is easy to see how waves emanating from multiple sources, such as drops of rainwater in still water, create inter-
ference patterns. But a single source of waves can create interference patterns with itself as a result of diffraction.
The Double Slit Experiment
A similar situation to the raindrops above occurs when straight waves strike a barrier containing two slits. These
waves are cut off everywhere except for where the waves that pass through the two slits. The medium in the slits
again acts as a point source to produce circular waves on the far side of the barrier.
As long as these two circular waves have the same wavelength, they interfere constructively and destructively in
a specific pattern. This pattern is called the wave interference patternand is characterized by light and dark
bands. The light bands are a result of constructive interference, and the dark bands occur because of destructive
interference.