In this paper we report both experimental and theoretical results on Rayleigh-Benard and Marangoni convection instability experiments in isotropic liquids and nematic liquid crystals. This instability is driven and controlled by medium absorbing a light radiation with a spatially periodical transverse structure. This instability has rather thermal than hydrodynamic nature because Prandtl number is much larger than one for our media. We study the competition of two mechanisms for reorientation of the liquid crystal molecules. Gravitational and thermocapillary hydrodynamic waves induced by traveling spatially periodical distribution of light intensity are observed and investigated. The stability of these waves is studied as well. We also study the influence of convective motions on the isotropic liquid- liquid crystal phase transition.