  We study an interface between two media separated by a strictly 2D sheet. We
show how the amplitude reflection coefficient can be modeled by that for an
interface where the 2D sheet has been replaced by a film of small but finite
thickness. We give the relationship between the 3D dielectric function of the
thin film and the 2D dielectric function of the sheet. We choose graphene and a
2D metallic sheet as illustrative examples. This approach turns out to be very
useful when treating graphene or graphene like sheets in non-planar structures
