  In the diffraction pattern produced by a half-plane sharp edge when it
obstructs the passage of a laser beam, two characteristic regions are
noticeable. There is a central region, where it can be noticed the diffraction
of laser light in the region of geometric shadow, while intensity oscillations
are observed in the non-obstructed area. On both sides of the edge, there are
also very long light traces along the normal to the edge of the obstacle. The
theoretical explanation to this phenomenon is based on the Fresnel-Kirchhoff
diffraction theory applied to the Gaussian beam propagation behind the
obstacle. Here we have supplemented this explanation by considering
electromagnetic flow lines, which provide a more complete interpretation of the
phenomenon in terms of electric and magnetic fields and flux lines, and that
can be related, at the same time, with average photon paths.
