  We present a general and useful method to predict the existence, frequency,
and spatial properties of gap states in photonic (and other) structures with a
gapped spectrum. This method is established using the scattering approach. It
offers a viewpoint based on a geometrical Fabry-Perot model. We demonstrate the
capabilities of this model by predicting the behaviour of topological edge
states in quasi-periodic structures. A proposition to use this model in Casimir
physics is presented.
