  The recent interest in the imaging possibilities of photonic crystals
(superlensing, superprism, optical mirages etc...) call for a detailed analysis
of beam propagation inside a finite periodic structure. In this paper, we give
such a theoretical and numerical analysis of beam propagation in 1D and 2D
photonic crystals. We show that, contrarily to common knowledge, it is not
always true that the direction of propagation of a beam is given by the normal
to the dispersion curve. We explain this phenomenon in terms of evanescent
waves and we construct a renormalized dispersion curve that gives the correct
direction.
