Refraction of light — Recall

What equation relates refractive index to the angles of incidence and refraction, and how is it used?

Refractive index n = sin i / sin r, where i is the angle of incidence and r is the angle of refraction, both measured from the normal, for light passing from a less dense medium into the denser medium. It is used to calculate n from measured angles, or to find an unknown angle given n.