With this simulation, you can explore the ideas of interference and diffraction resulting from waves passing through a single opening (the single slit); waves being emitted with equal amplitude in all directions from two sources (the double source), and waves passing through two closely spaced openings (the double slit). Note that the double-slit pattern is a combination of the single-slit and double-source patterns.
For the single slit that has a slit width a, the locations of the zeroes in the pattern (from destructive interference) are given by:
single-slit pattern, destructive interference: a sinθ = m λ, where m = 1, 2, 3, ...
For the double source, with a distance d between the sources, the locations of the zeroes in the pattern (from destructive interference) and interference maxima (from constructive interference) are given by:
double-source pattern, destructive interference: d sinθ = (m + 0.5) λ, where m = 0, 1, 2, ...
double-source pattern, constructive interference: d sinθ = m λ, where m = 0, 1, 2, ...
The double-slit pattern has the same equations as the double source, with the exception of what are known as missing orders. This is when bright regions of constructive interference that are predicted by the last equation above are not observed, because they coincide with zeroes in the pattern associated with the single slit equation. When d/a = 4, for instance, bright lines corresponding to m = 4, 8, 12, ... (multiples of d/a) are missing from the pattern, because neither slit sends out waves in those directions.