We present an experiment of modulation of the light propagating in a waveguide-liquid crystal (LC) planar stack, by an external light source. The results confirm that the molecular reorientation in the LC layer, induced by an external optical field, gives rise to a change in the refractive index which in turn modifies the propagation constants of the guided modes, so that the light can be partially leaked off the guide. It is shown how the modulation depends on the physical parameters of the materials and of the light.