Eliot Quataert, an astrophysicist at Princeton University, lectures at the Institute for Advanced Study on how polarized light from material near black holes can reveal whether rotational spin energy is being extracted from the black hole itself. He works through the physics of magnetized accretion flows and jets, explaining how polarization signatures encode information about magnetic field structure close to the event horizon. The talk connects this theoretical framework to recent observational efforts, including Event Horizon Telescope style polarimetry, that attempt to test models of energy extraction such as the Blandford-Znajek mechanism. Quataert walks through the underlying magnetohydrodynamics, the observational challenges of resolving these signals, and what current and future measurements can and cannot distinguish between competing models of jet power. Delivered as part of a seminar series on black holes from theory to observations, the lecture assumes some background in astrophysics but builds its argument step by step from physical principles to observational tests.