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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. A material's refractive index is the speed of light in a vacuum (c) divided by the speed of light within the material: the index is 1 for a vacuum, and higher for any actual material. Air's index is close to 1, water's is roughly 1.3, and those of various kinds of glass are 1.5 and higher. 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. They also depend upon the temperature of the material: these variations are small but detectable within the visible band, at the temperatures we experience. 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.