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Description
Light propagating through a perturbed spacetime could imprint the underlying gravitational waveform onto electromagnetic observables. We present a perturbative framework for polarized photon propagation in curved spacetimes, and derive an expression for the observable polarization-angle (PA) swing during Kerr ringdown, explicitly demonstrating its time-domain locking to the quasi-normal modes. We confirm this behavior using ray-tracing calculations for a broad class of photon trajectories. Photons grazing the strong-field region exhibit an achromatic, damped PA oscillation that tracks the ringdown, with a phase set by the mode's angular structure. The swing amplitude can reach ∼10∘ and leaves distinctive signatures in spatially resolved autocorrelations. These results open a potential window onto black hole ringdown.