MOKE: When linearly polarized light reflects from a magnetized surface, the polarization plane rotates by the Kerr angle θ_K ∝ M. The polar MOKE geometry is most sensitive: light along M (perpendicular to surface). Off-diagonal components of the dielectric tensor ε_xy = iQM (Q = magneto-optic coefficient) mix σ⁺ and σ⁻ circular polarizations, giving different reflectance → ellipticity + rotation. The hysteresis loop traces M(H) as field sweeps — coercivity H_c is the reversal field.