Snell's law is the scientific law of the refraction of light or other waves.
In optical science, Snell's law is about the relationship between the angles of incoming and outgoing light to the refractive indices of the mediums.
The law states that when light passes through different materials (for example from air to glass) the ratio of sines of the incidence (incoming) angle and the refraction (outgoing) angle does not change:
It can also be stated as "The ratio of the sines of the angles of incidence and refraction is equivalent to the ratio of phase velocities in the two media, or equivalent to the reciprocal of the ratio of the indices of refraction".
and be instead written as:
where #theta_1# and #theta_2# are the angles from the normal of the incident and refracted waves, respectively.
You can read more about it here and here.