Here's why:

* Vacuum: In a vacuum, there are no particles to interact with, so light can travel at its maximum speed, which is approximately 299,792,458 meters per second.

* Other mediums: When light travels through a medium like air, water, or glass, it interacts with the particles in that medium. These interactions slow the light down. The denser the medium, the slower the light travels.

This is why light bends when it passes from one medium to another, like from air to water – this is called refraction.