  We have performed broad-band zero-field and magneto-infrared spectroscopy of
the three dimensional topological insulator Bi$_{0.91}$Sb$_{0.09}$. The
zero-field results allow us to measure the value of the direct band gap between
the conducting $L_a$ and valence $L_s$ bands. Under applied field in the
Faraday geometry (\emph{k} $||$ \emph{H} $||$ C1), we measured the presence of
a multitude of Landau level (LL) transitions, all with frequency dependence
$\omega \propto \sqrt{H}$. We discuss the ramification of this observation for
the surface and bulk properties of topological insulators.
