Graphene has transformed the fields of plasmonics and photonics, and
become an indispensable component for devices operating in the
terahertz to mid-infrared range. Here, for instance, graphene surface
plasmons can be excited, and their extreme interfacial confinement
makes them vastly effective for sensing and detection. The rapid,
robust, and accurate numerical simulation of optical devices featuring
graphene is of paramount importance and many groups appeal to
Black-Box Finite Element solvers. While accurate, these are quite
computationally expensive for problems with simplifying geometrical
features such as multiple homogeneous layers, which can be recast in
terms of interfacial (rather than volumetric) unknowns. In either
case, an important modeling consideration is whether to treat the
graphene as a material of small (but non-zero) thickness with an
effective permittivity, or as a vanishingly thin sheet of current with
an effective conductivity. In this contribution we ponder the correct
relationship between the effective conductivity and permittivity of
graphene, and propose a new relation which is based upon a concrete
mathematical calculation that appears to be missing in the literature.
We then test our new model both in the case in which the interface
deformation is non-trivial, and when there are two layers of graphene
with non-flat interfacial deformation.