In recent years, investigation of light propagation with designed properties using the frequency bands of photonic crystals has opened new opportunities for shaping and moulding the flow of light for information technology, although much earlier applications of photonic crystals have focused on the frequency gaps for confining light. The principle of geometric optics can be extremely valuable here for using photonic crystals as effective media as have been proven so for usual optical materials. In this talk, I will present a geometric optics theory for light propagation in photonic crystals subject to slowly varying perturbations in space and time. The Poynting vector field, which traces the geometric optics rays, is not only governed by the photon energy dispersion but also involve three additional fields. They are the photon angular momentum, a dissipation dipole density, and various geometric (Berry) gauge fields in the extended phase space spanned by space-time and its reciprocal, momentum and frequency.