The analysis of reflections from thin films or dielectric materials can be approached by a matrix method that treats any thin-layer device as a cascade of sequential, zero-thickness reflecting thin-layer surfaces [J. Opt. Soc. Am. A 2, 1363 (1985)] . Our paper presents an alternative method for predicting the reflection/transmission characteristics of such dielectric films in a Fabry–Perot interferometer configuration based on a Gaussian-beam modal analysis within a scattering-matrix framework [in Proceedings of IEE 7th International Conference on Antennas and Propagation (IEE, 1991), Issue 15, p. 201.] We present and validate a scalar Gaussian-beam modal scattering-matrix approach using long-wavelength examples, where diffraction effects are important to model total transmission and reflection characteristics that also include a waveguide modal description of a corrugated horn. For optical beams the same technique is equally applicable, but diffraction is less severe within this framework. This approach is flexible and has many applications within laser optics and in far-infrared or submillimeter-instrumentation optical analysis, where it is possible to incorporate reflections in both waveguide and free space within the description of a whole system. To conclude and verify the accuracy of the technique, experimental measurements taken at $94\mathrm{GHz}$ are compared with theoretical predictions for a dielectric cavity of polyethylene sheets between corrugated source and detector antennas.