  In this note we consider the scattering of electromagnetic waves (governed by
the time-harmonic Maxwell equations) by a thin periodic layer of perfectly
conducting obstacles. The size of the obstacles and the distance between
neighbouring obstacles are of the same small order of magnitude $\delta$,
$\delta$ being small. By deriving homogenized interface conditions for three
model configurations, namely (i) discrete obstacles, (ii) parallel wires, (iii)
a wire mesh, we show that the limiting behaviour as $\delta\to0$ depends
strongly on the topology of the periodic layer, with full shielding (the
so-called "Faraday cage effect") occurring only in the case of a wire mesh.
