  Topological insulators can exhibit strong magnetoelectric effects when their
time-reversal symmetry is broken. In this Letter we consider the
magneto-optical Kerr and Faraday effects of a topological insulator thin film
weakly exchange-coupled to a ferromagnet. We find that its Faraday rotation has
a universal value at low-frequencies, $\theta_{\mathrm{F}} =
\mathrm{tan}^{-1}\,\alpha$ where $\alpha$ is the vacuum fine structure
constant, and that it has a giant Kerr rotation $\theta_{\mathrm{K}} = \pi/2$.
These properties follow from a delicate interplay between thin-film cavity
confinement and the surface Hall conductivity of a topological insulator's
helical quasiparticles.
