A periodic deformation can arise in a nematic liquid crystal (NLC)-cell with unidirectional planar alignment, when the NLC with positive susceptivity anisotropy is subjected to an external magnetic field. Such a periodic Frederiks distortion occurs, both in the case of strong- and weak-anchoring, if the ratio twist- to splay-elastic constants is lower than a critical value: the cell behaves as a diffraction grating. Here the optical phase retardation of a transmitted polarized light beam normally incident on the sample is studied in the approximation of small distortion, in order to find its dependence on the anchoring conditions. We show that the distortion periodicity goes as the 2nd-harmonic of the in-plane grating, close to Frederiks periodic threshold.