  We discuss here the validity of the small mass limit (the so-called
Smoluchowski-Kramers approximation) on a fixed time interval for a class of
semi-linear stochastic wave equations, both in the case of the presence of a
constant friction term and in the case of the presence of a constant magnetic
field. We also consider the small mass limit in an infinite time interval and
we see how the approximation is stable in terms of the invariant measure and of
the large deviation estimates and the exit problem from a bounded domain of the
space of square integrable functions.
