  Reconfigurable intelligent surfaces (RISs) are an emerging field of research
in wireless communications. A fundamental component for analyzing and
optimizing RIS-empowered wireless networks is the development of simple but
sufficiently accurate models for the power scattered by an RIS. By leveraging
the general scalar theory of diffraction and the Huygens-Fresnel principle, we
introduce simple formulas for the electric field scattered by an RIS that is
modeled as a sheet of electromagnetic material of negligible thickness. The
proposed approach allows us to identify the conditions under which an RIS of
finite size can or cannot be approximated as an anomalous mirror. Numerical
results are illustrated to confirm the proposed approach.
