In this seminar, we will explore observational signatures of nonsingular ultracompact objects regularized by nonlinear electrodynamics (NED). Due to the phenomenon of birefringence, photons of different polarizations propagate with respect to two distinct metrics, which manifests itself in the appearance of additional light rings surrounding the ultracompact object. I will outline the physical consequences of this result and illustrate them based on three regular black hole models commonly considered in the literature. Specifically, we will discover that nonsingular horizonless ultracompact objects sourced by NED possess an odd number of light rings and discuss the viability of NED as an effective description of their properties. In addition, we will compare the phase velocities of polarized light rays propagating in nonsingular NED geometries to the corresponding phase velocity in the Schwarzschild spacetime and demonstrate that regularizing the singularity by means of a theory that does not adhere to the Maxwell weak-field limit may lead to the emergence of acausal regions.

[Based on arXiv:2406.07957]