Etymology
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refraction (n.)

"act of refracting; state of being refracted," 1570s, from Late Latin refractionem (nominative refractio) "a breaking up," noun of action from past-participle stem of Latin refringere "to break up," from re- "back" (see re-) + combining form of frangere "to break" (from PIE root *bhreg- "to break"). According to Century Dictionary, "Almost exclusively restricted to physics" [1895].

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refract (v.)

"to bend or break the natural course of" (light, sound, heat, etc.), 1610s, back-formation from refraction, and in part from Latin refractus, past participle of refringere "to break up," from re- "back" (see re-) + combining form of frangere "to break" (from PIE root *bhreg- "to break"). Related: Refracted; refracting.

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refractory (adj.)

"stubborn, obstinate, perverse, resisting, unyielding," 1610s (earlier refractorious, 1550s, refractary, c. 1600), from Latin refractarius "obstinate, stubborn," from past participle stem of refringere "to break up" (see refraction). The notion is said to be "breaking back" all attempts to enforce obedience. The English spelling was corrupted on analogy of adjectives in -ory. Related: Refractorily; refractoriness.

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refrain (n.)

"regularly recurring phrase in a poem, chorus, or song," late 14c., refreine, from Old French refrain "chorus" (13c.), an alteration of refrait, a noun use of the past participle of refraindre "to repeat," also "to break off," from Vulgar Latin *refrangere "break off," alteration of Latin refringere "break up, break open" (see refraction) by influence of frangere "to break."

The word was further influenced in French by cognate Provençal refranhar "singing of birds, refrain." The notion is of something that causes a song to "break off" then resume. OED says not common before 19c. For "act of refraining," refraining (mid-14c.) and refrainment (1711) have been used.

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*bhreg- 

Proto-Indo-European root meaning "to break."

It forms all or part of: anfractuous; Brabant; bracken; brake (n.1) "stopping device for a wheel;" brake (n.2) "kind of fern;" brash; breach; break; breccia; breeches; brioche; chamfer; defray; diffraction; fractal; fraction; fractious; fracture; fragile; fragility; fragment; frail; frangible; infraction; infringe; irrefragable; irrefrangible; naufragous; ossifrage; refract; refraction; refrain (n.); refrangible; sassafras; saxifrage; suffragan; suffrage.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit (giri)-bhraj "breaking-forth (out of the mountains);" Latin frangere "to break (something) in pieces, shatter, fracture;" Lithuanian braškėti "crash, crack;" Old Irish braigim "break wind;" Gothic brikan, Old English brecan "to break."

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refractive (adj.)

"of or pertaining to refraction; serving or having the power to refract," 1670s, from Late Latin refractivus, or from refract + -ive.

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rainbow (n.)

"arc of prismatic colors formed by the refraction of light rays by drops of rain or vapor," Middle English rein-bowe, from Old English renboga; see rain (n.) + bow (n.). Common Germanic compound (Old Frisian reinboga, Old Norse regnbogi, Swedish regenbåge, Dutch regenboog, German Regenbogen). The American rainbow trout (1876) is so called for its resplendent colors. Old English also had scurboga "shower-bow."

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