  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, an
answer to the question "where does the beam emerge?" is given. Contrarily to
common knowledge, it is not always true that the shift of a beam is given by
the normal to the dispersion curve. This phenomenon is explained in terms of
evanescent waves and a renormalized diagram that gives the correct direction is
given.
