Snell's Law

Snell’s law states:

$n_1 \sin \theta_1 = n_2 \sin \theta_2$

where $n_1$ and $n_2$ are the refractive indices of the two media, and $\theta_1$ and $\theta_2$ are the angles measured from the normal.

When light goes from a less dense to a denser medium ($n_2 > n_1$), it bends toward the normal ($\theta_2 < \theta_1$).

At the critical angle, $\theta_2 = 90°$:

$\sin c = \frac{n_2}{n_1}$

Beyond the critical angle, total internal reflection occurs.