  Some optical experiments provide the easiest way to test quantum mechanical
predictions. Such a situation applies to a laser beam traversing a dielectric
structure. The dielectric structure mir- roring quantum mechanical potentials.
The simplest of these, studied in this paper, involves a single dielectric
block. We exhibit both by analytic and numerical calculations explicit examples
of total coherence and total incoherence phenomena. Which case appears depends
upon the angle of incidence of the laser beam and/or the dimensions of beam vs
block width. Unlike our previ- ous studies our calculations here are
three-dimensional, although, with suitable approximations, somewhat simpler
planar expressions can be derived.
