> The speed of light in glass or fiber-optic cable is significantly slower, at approximately 194,865 kilometers per second. The speed of voltage propagation in copper is 224,844 kilometres per second.

If I understand correctly, the speed of light in an electrical cable doesn't depend on the metal that carries current, but instead depends on the dielectric materials (plastic, air, etc.) between the two conductors?

If I’m interpreting what you’re asking correctly, yes. The velocity factor of a cable doesn’t spend on the metal it’s made of but rather the insulator material and the geometry of the cable.

For fibre the velocity factor depends on the refraction index of the fibre.

Huh? Maybe I'm completely misreading the question, but when they say fiber-optic cable, they do mean optic. It's not an "electrical cable"; there is no metal needed in optic communication cables (perhaps for stiffness or whatnot, but not for the communication)