The fastest thing in the whole universe is the speed of light in a vacuum. The speed is around 2.99 * [tex]10^{8}[/tex] . It is fastest in vacuum as vacuum does not contain any dust particles and is a hollow space so light travels faster. It also has a refractive index of 1 so it travels faster.
After vacuum light travels faster in air. Air has a refractive index of 1.0003.
After air, it travels faster in water. The speed is about 3/4 of its speed in a vacuum, or 225,000,000 meters per second. Speed slows in water because photons interact with atoms, which repeatedly absorb and re-emit them to the same direction. The refractive index of water is 1.33.
