Meaning of Snell's law

Snell's' law'

Pronunciation: (snelz), [key]
— Optics. Optics.
  1. the law that, for a ray incident on the interface of two media, the sine of the angle of incidence times the index of refraction of the first medium is equal to the sine of the angle of refraction times the index of refraction of the second medium.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.