Fibre-optic internet moves data at nearly the speed of light.
Why is it slower than light in a vacuum?

Question

Accepted Answer

The glass slows photons by about a third — they travel through fibre at roughly 200,000 km/s, not 300,000. — Refraction index of silica. Also, signals hop through routers and switches, each adding milliseconds. The theoretical vs practical gap is engineering, not physics.

Fibre-optic internet moves data at nearly the speed of light. Why is it slower than light in a vacuum?