'Refraction' is Latin for 'broken back' — light bending between media, conceived as the ray being 'broken.'
The change in direction of a wave, especially a light or sound wave, as it passes from one medium to another of different density. The measurement of this bending in optometry to determine corrective lens prescriptions.
From Late Latin 'refrāctio' (a breaking back, a breaking up), from 'refrangere' (to break up), a compound of 're-' (back, again) + 'frangere' (to break, shatter), from PIE *bʰreg- (to break). The same root 'frangere' gives English 'fracture,' 'fragment,' 'fragile,' 'infraction,' and 'fraction' — all sharing the idea of breaking into pieces. The prefix 're-' intensifies the backward deflection: light is conceptually 'broken back' at the boundary between media of differing optical density
The German word for refraction is 'Brechung' — literally 'breaking' — a direct calque of the Latin. This reveals the ancient intuition behind the scientific term: when light enters water at an angle and appears to bend, early observers imagined the light ray being broken. A stick half-submerged in water appears broken at the waterline — this everyday observation is the origin of both the physical concept and the word. The Latin 'frangere' (to break) also gave us 'fragile,' 'fraction,' 'fracture,' and 'infringe' — all words about things breaking.