Respuesta
$n_1 \sin(\theta_1) = n_2 \sin(\theta_2)$, where $n$ is the refractive index and $\theta$ is the angle with the normal.
Explicación
Total internal reflection occurs when light travels from a denser to rarer medium ($n_1 > n_2$) at an incident angle exceeding the critical angle $\sin(\theta_c) = \frac{n_2}{n_1}$.