GeoGebra

Single Slit Diffraction

This shows two rays, one from each edge of the slit (or aperture) to show how they interfere when they intersect.

You can adjust the slit width (a), the distance to the screen, and the position of intersection of the two rays (effectively, the angle of the ray from the normal to the slit)
It will then calculate the energy of the two superposed waves: Positive for a peak, negative for a trough, and zero for destructive interference, where the dark nodal lines will be seen.

This diagram should be used with the formula for calculating the angle of the m-th dark nodes in a single slit diffraction with light of wavelegth $\lambda$ and slit of width a.
$sin\theta = \frac {m \lambda}{ a}$,

It addresses some misconceptions present in many textbooks:

1. The rays only interfere when they intersect. Parallel rays can never interfere. It is true that in the real world the angle between the rays is very small, since the distance to the screen is typically 10000x the slit width. But they are still not parallel
   
2. The rays travel in straight lines. Many people think that the particles do some drunken lurching dance as they move along! The energy varies as the sine of the path (distance travelled) (wavelengths), but not the path itself. This has an important impact - when the two (energy) sine waves intersect, but the ray paths are NOT intersecting, it is merely an artefact of how we have represented the energy. Until the rays intersect, nothing interesting happens.

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