The ratio of the speed of light in a vacuum versus the speed of light through another medium. It is always greater than one, unless in a perfect vacuum, as the presence of matter retards the speed of light. When passing through a medium, long wavelengths refract less than short wavelengths, so the refractive index of a medium depends on the wavelength of light passed through it.
Related entry refractometer
Related category OPTICS AND OPTICAL PHENOMENA
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