  The macroscopic surface electric susceptibility determines the linear optical
properties of an insulating single-layer two-dimensional atomic crystal, and
can be expressed in terms of the microscopic polarizability of the atoms. We
compute the local electric field acting on a single atom, both for the static
and the dynamic case, as the superposition of the external applied electric
field and the fields generated by the induced dipoles in the crystal. We find
that, in the dynamic case, retardation effects dephase the local electric field
with respect to the incident one. This explains why the Fresnel coefficients of
a single-layer two-dimensional atomic crystal are intrinsically complex
quantities, even when a null macroscopic surface conductivity is assumed.
