When waves cross a boundary between two materials, there is a change of speed called **refraction.**

If the direction of the wave is at an angle other than the normal line, then a change in direction occurs.

The change in speed is described by the **refractive index**, n of the material.

**n is given by: n = c _{v} / c_{m}**

where c_{v} is the speed of light in a vacuum, and c_{m} the speed of light in the material in question. n is unitless.

The refractive index between two materials is: _{1}n_{2 }

It is the ratio of speed of light in material 1, to the speed of light in material 2, and is given by:

** _{1}n_{2} = c_{1}/c_{2} = n_{1}/n_{2}**

**Snell’s Law** states:

**sini /sinr = c _{1}/c_{2} = _{1}n_{2}**

From Snell’s law, the angle of refraction can be calculated if the refractive index of the two materials is known. The refractive index can be calculated from the sin of the angles of the light measured in the materials.