The speed of light depends on the medium through which it travels. In a vacuum, such as empty space, light travels at its maximum speed, which is approximately 299,792 kilometers per second (about 186,282 miles per second) in a vacuum, commonly denoted as 'c' in scientific equations. This is often referred to as the speed of light in a vacuum.
When light enters a transparent medium, such as water or glass, its speed decreases. This decrease in speed is due to the interaction between light and the atoms or molecules of the medium. As light passes through a medium, it interacts with the charged particles, primarily electrons, within the atoms or molecules.
These interactions cause the light to be absorbed and re-emitted by the particles, leading to a delay in the propagation of the light wave. This delay results in an overall reduction in the speed of light in the medium compared to its speed in a vacuum.
The extent to which light slows down in a medium is determined by the optical properties of the material, such as its refractive index. The refractive index is a measure of how much the speed of light is reduced when passing through a particular material.
Different materials have different refractive indices, and as a result, light will travel at different speeds through different transparent media. For example, light travels slower in water than in air, and it travels even slower in glass than in water.
It is important to note that light does not always slow down when passing through a medium. In some cases, such as when light passes from a medium with a higher refractive index to a medium with a lower refractive index, it can actually speed up. This phenomenon is known as refraction and is responsible for effects such as the bending of light when it passes through a lens.
In summary, the speed of light varies depending on the medium it travels through. It travels at its maximum speed in a vacuum and slows down when it passes through transparent media due to interactions with the particles in the medium, which causes delays in its propagation.