Snell's Law

Snell's Law describes how the direction of light changes when it moves from one medium to another. The two forms of the law are:

n₁ sin(θ₁) = n₂ sin(θ₂)

v₂ sin(θ₁) = v₁ sin(θ₂)

where:

n₁ and n₂ are the indices of refraction in media 1 and 2, respectively

v₁ and v₂ are the velocities of light in media 1 and 2, and

θ₁ and θ₂ are the angle from the normal of the plane dividing the 2 media and the 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.

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.