Light travels fastest through a vacuum. In a vacuum, such as outer space, where there is no material medium, light propagates at its maximum speed, which is approximately 299,792,458 meters per second (or about 186,282 miles per second) in a vacuum. This speed is often denoted by the symbol "c" and is considered a fundamental constant of nature, known as the speed of light in a vacuum.
When light travels through different materials, such as air, water, or glass, its speed is slower than in a vacuum. This reduction in speed is due to the interactions between light and the atoms or molecules of the material, which can cause absorption and scattering processes. The speed of light in a given material is characterized by the material's refractive index, which is a measure of how much the material slows down the speed of light compared to its speed in a vacuum.
In summary, light travels fastest through a vacuum, and its speed decreases when it passes through different materials.