Refractive index
![A ray of light being refracted in a plastic block.](/Images/godic/202412/19/Refraction_photo3510.png")
![Refraction of a light ray.](/Images/godic/202412/19/Refraction_at_interface.svg3511.png")
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![Diamonds have a very high refractive index of 2.42.](/Images/godic/202412/19/Brillanten3511.jpg")
In optics, the refractive index or index of refraction n of a material is a dimensionless number that describes how light propagates through that medium. It is defined as
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.