The Index of refraction of a substance is related to the speed of light in that it is a ratio of the speeds of light in that particular substance. The index of refraction IS defined to be the compariosn of the speed in a vacuum ot the speed of light in a substance. In other words, the speed of light is used in the equation to determine a substance's index of refraction in the following formula: speed of light in vacuum divided by the Speed of light in the substance.
