  We study observational signatures of nonsingular ultracompact objects
regularized by nonlinear electrodynamics. The phenomenon of birefringence
causes photons of different polarizations to propagate with respect to two
distinct metrics, which manifests itself in the appearance of additional light
rings surrounding the ultracompact object. We analyze the observational
consequences of this result and illustrate our findings based on three regular
black hole models commonly considered in the literature. We find that
nonsingular horizonless ultracompact objects sourced by nonlinear
electrodynamics possess an odd number of light rings and discuss the viability
of this effective description of their properties. In addition, we compare the
phase velocities of polarized light rays propagating in nonsingular geometries
sourced by nonlinear electrodynamics 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.
