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:

$n_{1}\sin {\theta _{1}}=n_{2}\sin {\theta _{2}}$
$v_{1}\sin {\theta _{1}}=v_{2}\sin {\theta _{2}}$

where:

$n_{1}$ and $n_{2}$ are the indices of refraction in media 1 and 2, respectively

$v_{1}$ and $v_{2}$ are the speeds of light in media 1 and 2

$\theta _{1}$ and $\theta _{2}$ 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.