  Chern-Simons (CS) forms generalize the minimal coupling between gauge
potentials and point charges, to sources represented by charged extended
objects (branes). The simplest example of such a CS-brane coupling is a domain
wall coupled to the electromagnetic CS three-form. This describes a
topologically charged interface where the CS form AdA is supported, separating
two three-dimensional spatial regions in 3+1 spacetime. Electrodynamics at
either side of the brane is described by the same Maxwell's equations, but
those two regions have different vacua, characterized by a different value of
the \theta-parameter multiplying the Pontryagin form F ^ F. The \theta-term is
the abelian version of the concept introduced by 't Hooft for the resolution of
the U(1) problem in QCD. We point out that CS-generalized classical
electrodynamics shows new phenomena when two neighboring regions with different
\theta-vacua are present. These topological effects result from surface effects
induced by the boundary and we explore the consequences of such boundary
effects for the propagation of the electromagnetic field in Maxwell theory.
Several features, including optical and electrostatic/magnetostatic responses,
which may be observable in condensed matter systems, like topological
insulators, are discussed.
