A computer simulation of the Huygens wavelet model of single slit diffraction.
The program breaks a single slit into a number of coherent wavelets and then calculates the electric field at a distant screen by summing the contribution of each wavelet, taking into account distance and phase.
The plot is the relative intensity, which is proportional to E^2.

wavelength=650 nm
screen distance ≈ 2m
screen width ≈ 1m
slit width (# of wavelengths):
num wavelets:
num calculations:
(It may take a while with a large number of calculations.)