Neutrinos are among the most abundant particles in the Universe and provide a unique probe of fundamental physics and extreme astrophysical environments. Although electrically neutral, their nonzero masses induce electromagnetic properties at loop level through Standard Model weak interactions. The dominant optical effect is parity-violating birefringence in asymmetric chiral neutrino media. This effect rotates the polarization plane of electromagnetic radiation propagating through neutrino-permeated plasmas. In this seminar, we discuss neutrino-induced birefringence in plasmas associated with active galactic nuclei (AGN) (arXiv:2601.15910 [hep-ph]). We show that the relative bulk motion between the neutrino background and the plasma introduces a directionality factor that gives rise to an anomalous frequency dependence distinct from conventional Faraday rotation. We evaluate the effect in AGN jets, relic cosmic-neutrino backgrounds, and accretion-disk plasmas. We also present a hyperaccretion scenario around stellar-mass black holes in which intense neutrino fluxes can significantly enhance the signal. Although the predicted rotation angles remain below current observational sensitivity, the distinctive spectral signatures provide a framework for probing neutrino asymmetries and astrophysical plasma properties independently of magnetic-field models.
