A new approximation scheme, designed to solve the covariant Maxwell equations inside a rotating hollow slender cavity (modelling a ring laser), is constructed. It is shown that for well-defined conditions there exist TE and TM modes with respect to the longitudinal axis of the cavity. A twisted mode spectrum is found to depend on the integrated Frenet torsion of the cavity and this in turn may affect the Sagnac beat frequency induced by a non-zero rotation of the cavity. The analysis is motivated by attempts to use ring lasers to measure terrestrial gravito-magnetism or the Lense–Thirring effect produced by the rotation of the earth.