  The r\^{o}le of the magnetoelectric effect upon optical reflectivity is
studied by adapting an electrodynamic-based model for a system composed by a 2D
metallic film in contact with an extended multiferroic material exhibiting weak
ferromagnetism. The well-known \emph{Nakayama's} boundary condition is
reformulated by taking into account the magnetoelectric coupling as well as an
externally applied magnetic field $\mathbf{B}$ in an arbitrary direction. It is
found that the reflectance shows strong fluctuations for incident radiation
close to the characteristic antiferromagnetic resonance frequency associated
with the multiferroic material in the THz regime. These results were verified
for a 10 nm metallic foil by using a finite element method (FEM) and the
Rouard's approach, for a wide range of wavelengths (0.1 - 5 mm), showing good
agreement with respect to Nakayama's outcome, for the particular material
BaMnF$_4$.
