index of refraction vs refractive index

index of refraction and refractive index both are nouns.

Word	Noun	Adjective	Verb	Adverb
index of refraction	Yes	No	No	No
refractive index	Yes	No	No	No

As nouns, index of refraction and refractive index are synonyms defined as:

index of refraction and refractive index: the ratio of the velocity of light in a vacuum to that in a medium

index of refraction (noun)	refractive index (noun)
the ratio of the velocity of light in a vacuum to that in a medium	the ratio of the velocity of light in a vacuum to that in a medium