  (Abridged) Context. To directly image rocky exoplanets in reflected
(polarized) light, future space- and ground-based high-contrast imagers and
telescopes aim to reach extreme contrasts at close separations from the star.
However, the achievable contrast will be limited by reflection-induced
polarization aberrations. While polarization aberrations can be modeled
numerically, such computations provide little insight into the full range of
effects, their origin and characteristics, and possible ways to mitigate them.
Aims. We aim to understand polarization aberrations produced by reflection off
flat metallic mirrors at the fundamental level. Methods. We used polarization
ray tracing to numerically compute polarization aberrations and interpret the
results in terms of the polarization-dependent spatial and angular
Goos-H\"anchen and Imbert-Federov shifts of the beam of light as described with
closed-form mathematical expressions in the physics literature. Results. We
find that all four beam shifts are fully reproduced by polarization ray tracing
and study the origin, characteristics, sizes, and directions of the shifts. Of
the four beam shifts, only the spatial Goos-H\"anchen and Imbert-Federov shifts
are relevant for high-contrast imagers and telescopes because these shifts are
visible in the focal plane and create a polarization structure in the PSF that
reduces the performance of coronagraphs and the polarimetric speckle
suppression close to the star. Conclusions. The beam shifts in an optical
system can be mitigated by keeping the f-numbers large and angles of incidence
small. Most importantly, mirror coatings should not be optimized for maximum
reflectivity, but should be designed to have a retardance close to 180{\deg}.
The insights from our study can be applied to improve the performance of
current and future high-contrast imagers, especially those in space and on the
ELTs.
