This form of Snell's law was actually published by Descartes as the law of Sines. Snell did discover the relationship but articulated it in a different way. Today it is the form used by Descartes that is called Snell's law.

The law is defined as:

n_{1} sinQ_{1} = n_{2} sinQ_{2}

Where n is the refractive index and Q the corresponding angles as shown.

The refractive index is the ratio of the speed of light in a vacuum to the speed of light in a given medium.

So, if the top part of the diagram is air, n_{1} is the speed of light in air and if the bottom part is glass, n_{2} is the speed of light in glass, both relative to the speed of light in a vacuum.

Snell and Descartes realized that when light went from one medium to another, the angles & refractive indexes of the media determined the path that light took. The relationship is a function of the sine of the angles.

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