  The degenerate case of normal incidence and reflection of an optical beam
(both paraxial and non-paraxial) at a plane isotropic dielectric interface,
which is azimuthally symmetric in terms of the momentum-spatial variation of
Fresnel coefficients but not in terms of the fundamental polarization
inhomogeneity of the incident field, requires in-depth analyses. In this paper,
we use the reflection and transmission coefficient matrix formalism to derive
an exact field expression of a normal-reflected diverging beam. The
availability of the exact field information allows controlled variations of the
system parameters, leading to significant dynamics of phase and polarization
singularities hitherto unanticipated in the literature. We carry out a detailed
exploration of these dynamics in our simulated system, and also verify them
experimentally by using an appropriate setup. We then use Barnett's formalism
to determine the associated orbital angular momentum (OAM) fluxes, leading to a
subtle interpretation and mathematical characterization of spin-orbit
interaction (SOI) in the system. Our work thus represents a non-trivial
unification of the most fundamental electromagnetic reflection/transmission
problem at a plane dielectric interface and the emerging areas of optical
singularity dynamics with their understanding in terms of OAM flux and SOI. The
normal-incidence--retro-reflection geometry being especially amenable to
applications, these beam-field phenomena are anticipated to have applications
in interface characterization, particle rotation/manipulation and other
nano-optical processes.
