Snell law for electromagnetic waves

For refractive indices $n_{-}$ and $n_{+}$,

Total internal reflection occurs when a wave travels from higher to lower refractive index above the critical incidence angle, leaving no propagating transmitted plane wave normal to the interface.

For refractive indices $n_{-}>n_{+}$, the critical angle satisfies $\sin {\theta }_c=n_{+}/n_{-}$. Above it, the transmitted normal wavenumber is imaginary and the transmitted field is an evanescent wave.