  In this work, we rigorously derive effective dynamics for light from within a
limited frequency range propagating in a photonic crystal that is modulated on
the macroscopic level; the perturbation parameter $\lambda \ll 1$ quantifies
the separation of spatial scales. We do that by rewriting the dynamical Maxwell
equations as a Schr\"odinger-type equation and adapting space-adiabatic
perturbation theory. Just like in the case of the Bloch electron, we obtain a
simpler, effective Maxwell operator for states from within a relevant almost
invariant subspace. A correct physical interpretation for the effective
dynamics requires to establish two additional facts about the almost invariant
subspace: (1) The source-free condition has to be verified and (2) it has to
support real states. The second point also forces one to consider a multiband
problem even in the simplest possible setting; This turns out to be a major
difficulty for the extension of semiclassical methods to the domain of photonic
crystals.
