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.
   [1913 Webster]

   {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.
      [1913 Webster]