Light travels slower in a fiber optic cable due to the refractive index of its materials, which slows light compared to its speed in a vacuum (300,000 km/s). In the fiber core, the higher refractive index reduces light's speed to about 200,000 km/s.
The design of fiber optics also uses total internal reflection to guide light down the cable. These repeated reflections increase the total distance the light travels and further increase the effective travel time.
