/ Dictionary / Index I index of refraction: Meaning and Definition of Find definitions for: in'dex of refrac'tion Pronunciation: [key] — Optics. Optics. a number indicating the speed of light in a given medium as either the ratio of the speed of light in a vacuum to that in the given mediumor the ratio of the speed of light in a specified medium to that in the given mediumSymbol: n Also called Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. index of leading indicators index plate Related Content Daily Word Quiz: fabricate Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language