Within the model of a Lorentz violating extension of the Maxwell sector of the standard model, modified light propagation leads to a change of the resonance frequency of an electromagnetic cavity, allowing cavity tests of Lorentz violation. However, the frequency is also affected by a material-dependent length change of the cavity due to a modified Coulomb potential arising from the same Lorentz violation as well. We derive the frequency change of the cavity taking both into account. The new effects derived are negligible for present experiments, but will be more pronounced in future tests using novel resonator materials.

7 pages, 2 figures, 1 table