In optics, the refractive index or index of refraction of a material is a dimensionless number n that describes how light propagates through that medium. It is defined as n = c/v where c is the speed of light in vacuum and v is the phase velocity of light in the medium. For example, the refractive index of water is 1.333, meaning that light travels 1.333 times faster in a vacuum than it does in water.

The picture illustrates the refraction of light through a plastic block.

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