Light travels at its maximum speed in a vacuum. In a vacuum, which is devoid of any material medium, light travels at approximately 299,792,458 meters per second (or about 186,282 miles per second) in a phenomenon commonly denoted as "c" in physics.
However, when light travels through different substances, such as air, water, or glass, its speed can be reduced due to interactions with the atoms or molecules of the medium. The reduction in speed is a result of the light being absorbed and re-emitted by these particles, leading to a slower overall propagation.
The speed of light in a medium is characterized by the medium's refractive index, which represents how much the medium can slow down the speed of light compared to its speed in a vacuum. The refractive index varies for different materials. For example, the refractive index of air is very close to 1, while water has a refractive index of about 1.33, and glass can have a refractive index ranging from approximately 1.5 to 1.7, depending on its composition.
In summary, light travels fastest in a vacuum, where it achieves its maximum speed of approximately 299,792,458 meters per second. In other materials, such as air, water, or glass, light travels slower due to interactions with the atoms or molecules of the medium, resulting in a reduced speed determined by the medium's refractive index.