  A topological insulator is classically modeled as an isotropic
dielectric-magnetic with a magnetoelectric pseudoscalar $\Psi$ existing in its
bulk while its surface is charge-free and current-free. An alternative model is
obtained by setting $\Psi\equiv0$ and incorporating surface charge and current
densities characterized by an admittance $\gamma$. Analysis of plane-wave
reflection and refraction due to a topological-insulator half space reveals
that the parameters $\Psi$ and $\gamma$ arise identically in the reflection and
transmission coefficients, implying that the two classical models cannot be
distinguished on the basis of any scattering scenario. However, as $\Psi$
disappears from the Maxwell equations applicable to any region occupied by the
topological insulator, and because surface states exist on topological
insulators as protected conducting states, the alternative model must be
chosen.

Optics