Light in glass
A fibre is thinner than a hair. Light bounces off its walls and cannot escape (total internal reflection). At first little light came out of the glass because it was not pure enough. Kao realised the problem was impurities, not the principle.
Submarine cables
Hundreds of undersea fibre cables connect the continents. Most international internet traffic flows through them; satellites carry only a small share. Big firms such as Google and Meta now build their own cables.
Fibre at home
Fibre to the home (FTTH) brings gigabit and faster speeds with low latency. It is more stable than Wi-Fi and mobile connections. In Slovakia operators are gradually extending it beyond the big cities.