refractive index vs ratio

refractive index and ratio both are nouns.

Word	Noun	Adjective	Verb	Adverb
refractive index	Yes	No	No	No
ratio	Yes	No	No	No

As nouns, ratio is a hypernym of refractive index; that is, ratio is a word with a broader meaning than refractive index:

refractive index: the ratio of the velocity of light in a vacuum to that in a medium
ratio: the relative magnitudes of two quantities (usually expressed as a quotient)

refractive index (noun)	ratio (noun)
the ratio of the velocity of light in a vacuum to that in a medium	the relation between things (or parts of things) with respect to their comparative quantity, magnitude, or degree
	the relative magnitudes of two quantities (usually expressed as a quotient)