(1) The deflection of a ray of light when it passes from one medium into another of different optic density; in passing from a denser into a rarer medium it is deflected away from a line perpendicular to the surface of the refracting medium; in passing from a rarer to a denser medium it is bent toward this perpendicular line. (2) The act of determining the nature and degree of the refractive errors in the eye and correction of the same by lenses. SYN: refringence. [L. refractio (see refract)]