In 1995, Hadley formulated an elegant effective-index model to describe the formation of transverse modes in optical cavities (Hadley 1995 Opt. Lett. 20 1483–5). We apply this model to Fabry–Perot cavities and discuss its limitations, using the well-known paraxial solutions of these cavities as reference. We also introduce a new model, which we call the phase-plate model, that has less limitations and yields the correct first-order correction to the resonance frequencies for longer cavities. The analysis uses scalar optical fields in the paraxial limit.