Light from other element pairs symmetric to the centerline also arrive in phase. If light from symmetric elements near each edge of the slit travels to the centerline of the slit, as indicated by rays 1 and 2 above, their light arrives in phase and experiences constructive interference. The spreading of the waves into the area of the geometrical shadow can be modeled by considering small elements of the wavefront in the slit and treating them like point sources.
The phenomenon of diffraction involves the spreading out of waves past openings which are on the order of the wavelength of the wave. This is an attempt to more clearly visualize the nature of single slit diffraction. Fraunhofer Single Slit Diffraction Fraunhofer Single Slit
