Optical fiber

An optical fibre is a hair-thin thread of ultra-pure glass that carries light the way a copper wire carries electricity. Its trick is a core surrounded by a slightly lower-index cladding: light striking the boundary at a shallow angle is bounced straight back in by total internal reflection, so a pulse launched at one end ricochets down the core and can travel kilometres without escaping — a pipe for photons. Because it ships information on light instead of electrons, one fibre can carry far more data than any cable and ignores electrical noise entirely.

Modern long-haul fibre is astonishingly clear: a beam loses only about 0.2 dB per kilometre at 1550 nm, meaning the light can run ~80 km before it needs amplifying — try that with copper. A single strand routinely carries terabits per second by stacking dozens of laser colours side by side (wavelength-division multiplexing). The whole internet's intercontinental traffic rides on bundles of these fibres lying on the seabed, each one thinner than a strand of spaghetti.