Snell's Law
Snell's Law describes how the direction of light changes when it moves from one medium to another. What Snell's Law says is that when light moves from a medium with a high index of refraction to a medium with a low index of refraction it will bend away the normal of the plane dividing the two media. Conversely, when light moves from a medium with a low index of refraction to a medium with a high index of refraction it will bend towards from the normal dividing the two media.
The two forms of the law are:
where:
and 
are the indices of refraction in media 1 and 2, respectively
and 
are the speeds of light in media 1 and 2
and 
are the angles measured from the normal and the respective ray.
The index of refraction of a material is inversely proportional to the speed of light in that material, i.e. light will move slower in a material with a high index of refraction than it will in a material with a low index of refraction.
