to deflect (e.g. light or another wave motion) by refraction from one straight path to another when passing from one medium, e.g. glass, to another, e.g. air, in which the velocity is different
to determine the refracting power of (e.g. a lens)
[Latin refractus, past part. of refringere to break open, break up, refract, from re- + frangere to break]