refractiveness vs refractivity

refractiveness and refractivity both are nouns.

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

As nouns, refractiveness and refractivity are synonyms defined as:

refractiveness and refractivity: the physical property of a medium as determined by its index of refraction

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