Re-frăct′ive, a.

a. [Fr. réfractif. See REFRACT.] Serving or having power to refract or turn from a direct course; pertaining to refraction; as, refractive powers.

Refractive index, or index of refraction, of a substance, the ratio of the sine of the angle made by a ray of light with a perpendicular to the refracting surface before entering the substance, to the sine of the angle made by it with the perpendicular after entering the medium; or the quotient arising from dividing the sine of the angle of incidence by the sine of the angle of refraction.—Absolute refractive index, the quotient thus arising when the ray passes into the substance from a vacuum.—Relative refractive index of two substances, the same quotient or ratio when the ray passes from one substance into the other.