Figure 19.  The enigma of the Poisson spot. (Figures 19 A-C adapted from Kelly et al.)

A.Conditions associated with the production of a Poisson spot from a point light source.

B.Young’s explanation of diffraction effects.  Within the geometric shadow interference patterns are created by the interactions of secondary rays that are scattered by the obstruction.  Outside of the geometrical shadow diffraction patterns are created by interference between scattered rays and primary rays.  Compare Wood’s adaptation of Young’s proposal, Fig. 5-B, with Figs. 6 and 17-A in which it is proposed that activated pulsons create the conditions necessary for “scattering” effects.

C.Fresnel’s proposal.  Fresnel’s explanation of diffraction effects based on an adaption of Huygens’ principle.  For this example, a portion of the wave front of π radians that are divided by paths which differ in length by λ/2 to the midpoint of the geometric shadow.  Fresnel considered how waves from these zones would interfere to calculate the diffraction effects of the mask shown in Fig. 18-A.  Fresnel used this decomposition to make predictions about the intensity, width, and positions of the fringes of the resulting diffraction patterns.  Although the Fresnel-Huygens model is useful in accounting for interference effects, is it consistent with the “ring of fire” associated with the periphery of an illuminated disc?  How do the points on the wave front “know” at what wave lengths and when to perpetuate as spherical wavelets?

D.Photon model.  Photons emitted by the source create an excitation distribution beyond the barrier that accounts for the Poisson spot.  Because of the uncertainty associated with locations of in-transit photons, a simple pictorial representation of the process is not possible.  The photon model may be utilized to account for Taylor’s experiment (Fig. 8); whereas the models of Young and Fresnel do not.

E.Pulson model.  The pulson model as developed herein may be utilized to account for the Poisson spot (Fig. 17), Taylor’s experiment (Fig. 8), and the luminous edge surrounding a disc illuminated by a point source.  It also predicts a reflective Poisson spot (Fig. 17 B) that Fresnel’s model would not account for.  It has not been directly subjected to rigorous mathematical analyses, although initial boundary conditions represent essentially the same starting premises as Young’s proposal that has received considerable mathematical support.