refractive vs refractivity

refractive is not a noun while refractivity is a noun.

Word	Noun	Adjective	Verb	Adverb
refractive	No	Yes	No	No
refractivity	Yes	No	No	No

refractive (noun)	refractivity (noun)
	the physical property of a medium as determined by its index of refraction

refractive (adjective)	refractivity (adjective)
of or relating to or capable of refraction
capable of changing the direction (of a light or sound wave)