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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 the light is passing through. The refractive index quantifies the speed reduction as compared to the speed of light in a vacuum (c): the higher the refractive index, the slower is the speed of light through the material. 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. The varying effects by wavelength are termed dispersion, specifically chromatic dispersion.