Origins
The word 'refraction' entered English in the early seventeenth century from Late Latin 'refrāctiō' (a breaking back), derived from the Latin verb 'refringere' (to break up, to break back), a compound of 're-' (back, again) and 'frangere' (to break). The PIE root is *bhreg- (to break), which also gave English 'break' through the Germanic branch. The underlying metaphor is vivid: when a ray of light passes from one medium to another — from air into water, from air into glass — it changes direction, and this change was conceptualized as the ray being broken.
The observation that light bends when entering water is ancient. A straight stick partially submerged in water appears to bend or break at the waterline. Ptolemy studied this phenomenon in the second century CE, measuring the angles of incidence and refraction for light passing from air into water and into glass. But the precise mathematical law governing refraction was not established until the early seventeenth century, when Willebrord Snellius (Snell) in the Netherlands formulated what is now known as Snell's law: the ratio of the sines of the angles of incidence and refraction is constant for a given pair of media. This constant is the refractive index.
Snell's law, published posthumously and independently formulated by René Descartes, was fundamental to the development of optics. It explained why lenses work: the curved surface of a lens causes light rays entering at different points to refract by different amounts, converging them to a focus. Without understanding refraction, the design of telescopes, microscopes, eyeglasses, and cameras would be purely empirical — trial and error rather than mathematical design.
Development
The physics of refraction involves the change in the speed of light as it enters a different medium. Light travels fastest in a vacuum (about 300,000 kilometers per second) and slower in denser media — about 225,000 km/s in water, about 200,000 km/s in glass. When a light ray crosses the boundary between media at an angle, one side of the wavefront slows down before the other, causing the wave to pivot — to change direction. This is refraction. The analogy often used is a line of marching soldiers turning because one end of the line hits mud and slows down while the other end is still on pavement.
Atmospheric refraction affects what we see every day. The sun appears above the horizon when it is actually below it, because the atmosphere refracts sunlight around the curvature of the earth. Sunsets are prolonged by several minutes due to atmospheric refraction. Mirages — the shimmering illusions of water on hot roads or distant cities floating above the desert — are caused by the refraction of light through layers of air at different temperatures and densities.
In optometry, 'refraction' refers specifically to the eye exam that determines a patient's corrective lens prescription. The optometrist measures how the eye refracts light and calculates the lens power needed to bring the focal point precisely onto the retina. Myopia (nearsightedness) means the eye refracts light too strongly, focusing it in front of the retina. Hyperopia (farsightedness) means the eye refracts too weakly. The corrective lens adjusts the refraction so the image falls sharply on the retina.
French Influence
The Latin root 'frangere' (to break) produced a large family in English. 'Fraction' (a broken part of a whole), 'fracture' (a break in a bone or material), 'fragment' (a piece broken off), 'fragile' (easily broken), 'frail' (weak, breakable, through Old French), 'infringe' (to break into, to violate), and 'diffraction' (the breaking apart of waves as they pass through an opening or around an edge) are all siblings. The German calque 'Brechung' (breaking) for 'refraction' preserves the same image.
The concept of refraction has extended into figurative language, though less commonly than 'reflection.' To 'refract' an idea is to pass it through a medium that changes its direction — a culture, a personality, an ideology that bends the original thought into something new. Literary criticism speaks of how a writer 'refracts' reality through language, producing not a transparent window onto the world but a bent, transformed, and often more revealing image.