  Reconfigurable reflectors have a significant potential in future
telecommunication systems, and approaches to the design and realization of full
and tunable reflection control are now actively studied. Reflectarrays, being
the classical approach to realization of scanning reflectors, are based on the
phased-array theory (the so-called generalized reflection law) and the physical
optics approximation of the reflection response. To overcome the limitations of
the reflectarray technology, researchers actively study inhomogeneous
metasurfaces, using the theory of diffraction gratings. In order to make these
devices tunable and fully realize their potential, it is necessary to unify the
two approaches and study reconfigurable reflectors from a unified point of
view. Here, we offer a basic tutorial on reflectarrays and reflecting
metasufaces, explaining their common fundamental properties that stem from the
diffraction theory. This tutorial is suitable for graduate and post-graduate
students and hopefully will help to develop more deeper understanding of both
phased arrays and diffraction gratings.
