Before introducing Snell's Law we first must know how fast light will travel in a specific substance. The bigger a change in the speed of the light the more it will bend.
The index of refraction tells us how fast light is traveling in the substance by comparing the speed of light in the substance to the speed of light in a vacuum.
Looking at this equation you can tell that n can never be lower than 1. This is because light can never travel faster than it does in a vacuum. The higher the value for n the slower the light is moving and the higher potential it has to change direction when entering the substance.
Now that we can describe the relative speed of light in a substance, we can set up an equation to predict the degree which light will change direction when entering a substance.
All angles are measured from a normal line drawn perpindicular to the boundary between the two substances. To know how much light will bend we merely need to know the index of refraction between the two substances and the incoming angle.
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