Total internal reflection can occur only when

Total internal reflection occurs when a light ray travels from an optically denser medium to an optically rarer medium.

According to Snell's law, $\mu_1 \sin i = \mu_2 \sin r$.

For total internal reflection to occur, the angle of refraction $r$ must reach 90^{}^{{}^{\circ}} for an angle of incidence i < 90^{}^{{}^{\circ}}.

This implies \sin i = \dfrac{\mu_2}{\mu_1} \sin 90^{}^{{}^{\circ}} = \dfrac{\mu_2}{\mu_1}.

Since $\sin i < 1$, we must have $\mu_2 < \mu_1$.

Thus, light must travel from a medium of higher refractive index (denser) to a medium of lower refractive index (rarer).