Path length differs from velocity. The speed of light in a medium varies according to the refractive index of the medium. [1] Free space (vacuum) has an RI of

Path length differs from velocity. The speed of light in a medium varies according to the refractive index of the medium. [1] Free space (vacuum) has an RI of 1.0. Photons move fastest in free space, about 300,000km/sec. Water has an RI of ~1.33, meaning photons move through water at about 1/1.33, or about 75% of the free space velocity of light. RI of optical fiber is often ~1.4. So while there may be path length increase due to off-axis propagation and bouncing, the actual photon speed in the medium is 1/1.4, or about 71% of the 300,000km/sec speed of light in vacuum. [1] https://en.wikipedia.org/wiki/Refractive_index https://en.wikipedia.org/wiki/Refractive_index