  We study the main properties of the one-loop vacuum polarization function for
massless spinor QED4 in a slab, namely, with fields defined on M in R^(3+1),
such that M = {(x0, ..., x3)|0 <= x3 <= epsilon}, and bag-like boundary
conditions on the boundary. We evaluate the induced charge density and current
due to an external constant electric field normal to the boundary; we also
study the effective action for a purely transverse field, identifying its
epsilon-dependent contribution.
