  We demonstrate that the tuning of the optical properties of a photonic
crystal infiltrated with liquid crystal can be calculate using the Von-Laue
diffraction condition. We present a simple formula to predict the shift of the
stop band for all the diffraction orders using an effective index of the
composite structure. We consider that our formula is useful to determine in a
simple manner the shift of the optical properties of tunable photonic crystals.
We compare the accuracy of our method with calculations obtained with the Plane
Wave Method.
