How does fibre optic cable work?

Each fibre has a glass core surrounded by cladding with a slightly lower refractive index. When light hits the boundary at a shallow angle, it reflects completely back into the core rather than escaping, a process called total internal reflection. Lasers switch on and off billions of times per second to encode data as light pulses, and different wavelengths can carry separate signals at the same time. Modern fibre loses so little light that signals can travel tens of kilometres before needing amplification. Undersea fibre cables carry the vast majority of international internet traffic.