In optics the refractive index or index of refraction n of a substance (optical medium) is a dimensionless number that describes how light, or any other radiation, propagates through that medium.
It is defined as
n=c/v
where c is the speed of light in vacuum and v is the speed of light in the substance.
For example, the refractive index of water is 1.33, meaning that light travels 1.33 times slower in water than it does in vacuum.
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