  A Chern-Simons theory in 3D is accomplished by the so-called $\theta$-term in
the action, $(\theta/2)\int F\wedge F$, which contributes only to observable
effects on the boundaries of such a system. When electromagnetic radiation
interacts with the system, the wave is reflected and its polarization is
rotated at the interface, even when both the $\theta$-system and the
environment are pure vacuum. These topics have been studied extensively. Here,
we investigate the optical properties of a thin $\theta$-film, where multiple
internal reflections could interfere coherently. The cases of pure vacuum and a
material with magneto-electric properties are analyzed. It is found that the
film reflectance is enhanced compared to ordinary non-$\theta$ systems and the
interplay between magneto-electric properties and $\theta$ parameter yield film
opacity and polarization properties which could be interesting in the case of
topological insulators, among other topological systems.
