Light travels fastest through a vacuum. In a vacuum, such as outer space, where there is no matter or particles to interact with, light can travel at its maximum speed, which is approximately 299,792 kilometers per second (or about 186,282 miles per second) in a vacuum. This speed is commonly denoted as "c" and is the fundamental constant of the speed of light.
When light interacts with matter or travels through a medium other than a vacuum, such as air, water, or glass, its speed is generally reduced. The speed of light in a given medium is determined by the properties of that medium, such as its refractive index, which quantifies how much the medium slows down the propagation of light compared to its speed in a vacuum.
For example, in air, light travels slightly slower than its speed in a vacuum, but the difference is negligible for most practical purposes. In other materials like water or glass, the speed of light is further reduced due to their higher refractive indices.