Here's why:

* Vacuum: In a vacuum, there's no matter to interact with, so light travels at its maximum speed, which is approximately 299,792,458 meters per second. This is often rounded to 300,000,000 meters per second for easier calculations.

* Air: Air, though less dense than water or solids, still contains molecules that light can interact with. These interactions cause the light to slow down slightly. The speed of light in air is only about 0.03% slower than in a vacuum.

While this difference might seem small, it's significant enough that scientists consider the speed of light in a vacuum to be the "true" speed of light.