/ Dictionary / Index R relative index of refraction: Meaning and Definition of Find definitions for: rel'ative in'dex of refrac'tion Pronunciation: [key] — Optics. Optics. See under Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. relative impediment relatively Related Content Daily Word Quiz: eccentric Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language