Wormholes, spacetimes with a nontrivial topology, can exhibit multiple light rings. The existence of stable light rings can lead to the emergence of trapped modes around the throat of these objects, resulting in echoes in their ringdown profile and spectral lines in the absorption spectra. We investigate the scattering of light rays and massless scalar fields by thin-shell wormholes formed by gluing two (not necessarily identical) Reissner-Nordström spacetimes in the context of the Palatini $f(\mathcal{R})$ theory. These asymmetric wormholes can have different numbers of light rings on each of their sides; for instance, one side can exhibit a pair of light rings (one unstable and the other stable) while the other side exhibits a single unstable light ring. The (asymmetric) light ring structure of these wormholes may lead to stationary points in the deflection function, and consequently to the existence of caustics and rainbow scattering in the spacetime. In order to probe these phenomena in the wave dynamics, we investigate the scattering of massless scalar waves by these objects. Moreover, we discuss the impact of trapped modes on wave scattering, showing that near the trapped modes the partial scattering cross section presents a Fano resonance.

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• Alternative gravity theories
• Gravitation