A model for the propagation of an optical field in a planar nematic cell is presented. The effects produced by the hydrodynamic couplings due to backflows and to an externally induced flow are explicitly taken into account. It is shown that when the resulting flow is a planar Couette flow, the liquid crystal distortion induced by the field gives rise to an index-gradient which generates a wave guiding effect that concentrates the ray trajectories and the electromagnetic energy density into the central part of the cell.