We report on the influence of backward propagating waves on the twist optical Fréedericksz transition when a linearly polarised light impinges at normal incidence on a nematic liquid crystal film with planar alignment. We show that the reorientation threshold oscillates as a function of the optical thickness of the nematic layer. The amplitude of these oscillations strongly depends on the refractive index changes at the film boundaries and is shown to be related to interferential effects between forward and backward propagating waves that arise from unavoidable dielectric permittivity tensor mismatch and the film boundaries.