Dispersion is when visible light is separated into the colors that make up the light. We’ve already seen how optical density is a measure of how much a medium slows down light that travels through it. The index of refraction depends on the frequency of the light.

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The index of refraction is 1.51 for red light and 1.53 for violet, which means that as light goes through glass, it slows down the violet light just a little it more than it does the red. Because of this, prisms can unmix light by dispersion because the prism has two (or more) boundaries where this effect adds to separate white light into its colors.

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