An Etymological Dictionary of Astronomy and Astrophysics

1. Optics: The change of direction which a beam of light undergoes as it enters a medium of different → refractive index.
   
2. Acoustics: The change in direction in sound waves on reaching the boundary between two media.
   
3. Seismology: The bending of a seismic wave as it enters a material of different density.

Etymology (EN): From L.L. refractionem (nominative refractio) “a breaking up,” from L. refractus, p.p. of refringere “to break up,” from re- “back” + combining form of frangere “to break,” from PIE base *bhreg- “to break” (cf. Goth. brikan, O.E. brecan “to break;” Lith. brasketi
“crash, crack”).

Etymology (PE): Šekast, past stem of šekastan “to break, split;” Mid.Pers. škastan “to break;” Av. scind-, scand “to beak, cleave;”
Proto-Iranian *skand- “to break, cleave;” PIE sken- “to cut off.”