"index of refraction." YourDictionary, n.d. Web. 19 January 2019. <https://www.yourdictionary.com/index-of-refraction>.
index of refraction. (n.d.). Retrieved January 19th, 2019, from https://www.yourdictionary.com/index-of-refraction
A measure of the extent to which a substance slows down light waves passing through it. The index of refraction of a substance is equal to the ratio of the velocity of light in a vacuum to its speed in that substance. Its value determines the extent to which light is refracted when entering or leaving the substance.