Fiber optics
Definition
Hair-thin strands of glass that carry information as pulses of light, bouncing the light along inside the fiber. They form the backbone of the internet.
Technical definition
Waveguides that confine light by total internal reflection: a high-refractive-index core surrounded by lower-index cladding keeps light rays reflecting inside the core over kilometers. Digital data is encoded as modulated light pulses, typically infrared.
Example
A single fiber pair can carry terabits per second — millions of simultaneous video calls — because light's enormous frequency allows huge bandwidth. Undersea fiber-optic cables carry over 99% of intercontinental internet traffic.
The light in a fiber does not travel at the full speed of light in vacuum — glass slows it to about two-thirds of c. And fiber does not amplify the signal itself; light weakens with distance and needs repeater stations, which is why transoceanic cables have powered repeaters roughly every 100 km.