Title text:
Imagine you could ride alongside a sound wave. It would probably be pretty cool, right? We're putting in a departmental budget request to buy a really fast plane so we can check it out.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3238/
Speed of light in fiber is ~0.66c₀. Light is slowed down a bit by air, too. So...
That is not the speed of light through fiber. That is the speed of light bouncing at diagonal angles as it travels through fiber.
It appears to travel slower because it's not traveling in a straight line. But I promise, every individual photon is traveling at exactly c. Photons emmited simultaneously will not all arrive at a destination at the same time, but this isn't because they're traveling at different speeds, it's because they're taking different paths, reflecting and refracting slightly differently.
Do the individual photons slow down? No. But those photons get absorbed by atoms along the way and then a new photon of light is emitted (nearly, but not literally, instantly) which then continues along it's merry way at C until it encounters another atom. What slows down is the net speed of transport through a given medium.