What is "relative refractive index"

Refractive \Re*fract"ive\ (r?*fr?kt"?v), a. [Cf. F. r['e]fractif. See Refract.] Serving or having power to refract, or turn from a direct course; pertaining to refraction; as, refractive surfaces; refractive powers.

Refractive index. (Opt.) See Index of refraction, under Index.

Absolute refractive index (Opt.), the index of refraction of a substances when the ray passes into it from a vacuum.

Relative refractive index (of two media) (Opt.), the ratio of the sine of the angle of incidence to the sine of the angle of refraction for a ray passing out of one of the media into the other.