# Snell's law

$\frac{\sin\theta_1}{\sin\theta_2} = \frac{v_1}{v_2} = \frac{n_2}{n_1}$
with each $\theta$ as the angle measured from the normal of the boundary, $v$ as the velocity of light in the respective medium (SI units are meters per second, or m/s) and $n$ as the refractive index (which has no units) of the medium. The index of refraction of a vacuum is 1 and the velocity of light in a vacuum is $c$. When a wave passes the material where the index of refraction is n, the velocity of the wave becomes $\frac{c}{n}$.