Figure 12.Explanation for single slit interference according to Huygens principle under Fraunhofer conditions. The slit width (w) is divided into a series of wavelets sources (generators) that are paired according to their ability to interfere.

A.When the screen is at a great distance the ray paths of wavelets are treated as if they travel parallel paths.

B. Destructive interference for the first minima is considered to occur at a distance screen where the centerline point of wavelet production created at an angle is ½ wave length out of phase with the production of edge wavelets based on the distances of their ray paths to the target.  Under this condition each generator on respective sides of the midpoint is considered paired with a generator with which it is out of phase.  This proposal requires that each generator point of wavelets must exhibit coherent frequency with the mother signal.  As originally proposed, Huygens Principle was independent of wavelength considerations.

C.As the angles of rays relative to the centerline are increased beyond the first minimum a pattern of maxima and minima is generated.  Whereas the patterns for sequential minima occur in a balanced fashion (C-1), the maxima do not and hence are created at decreasing levels of intensity (C-2).  Thus the light intensity in the secondary maxima are considerable less than along the centerline (Fig.10-A*).  The pulson model accounts for the intense central peak by the constructive interference between the primary signal and the secondary edge signals; whereas, the interference patterns beyond the central peak are generated by the mutual interactions of secondary edge pulsons exclusive of the primary signal (Fig. 9).