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A material's refractive index is a measure of the refraction it produces (an effect on light passing through it at a given wavelength), used to determine the angle at which light is redirected when entering or exiting the material, as per Snell's law. The redirection occurs because of a change in the speed of light, which differs depending up the material that the light is passing through. The refractive index quantifies the speed reduction as compared to the speed of light in a vacuum (c): a slower speed of light through the material indicates a higher refractive index. A lens makes use of this redirection occurring at its surface, e.g., to magnify. Refractive indexes depend upon both the material and the wavelength and can be determined for other EMR as well as visible light. A material's variation of refractive index by wavelength means light of different wavelengths is redirected by different angles, a phenomenon termed dispersion, specifically chromatic dispersion.