When a time harmonic electromagnetic wave impinges on a slab a certain portion of the wave creates heat within the slab through dipolar and ohmic heating. The electrical and thermal properties of the material dictate the dynamical nature of the heating process, as well as the steady-state temperature profile. The material considered here is a slab of fluid. We consider the case where the fluid is bounded by thin rigid layers of transparent material. The steady-state heating profile governs the types of convective motions that can occur and also affects the stability characteristics of temperature, pressure and velocity perturbations introduced in the slab. The main objective here is to examine such stability characteristics, initially in the linear regime. Both rigid-rigid and rigid-free configurations are considered.