Light does not actually slow down in a medium in the way that, for example, a car would slow down when entering a dense material like mud. Instead, the apparent slowing down of light in a medium is due to its interaction with the atoms or molecules in that medium.
In a vacuum, light travels at a constant speed, which is approximately 299,792,458 meters per second (or about 186,282 miles per second). This speed is often referred to as the "speed of light in a vacuum" and denoted by the symbol 'c'. However, when light passes through a medium, such as water, glass, or air, its speed is slightly reduced.
The reason for this reduction in speed is that the electromagnetic waves that make up light interact with the charged particles (electrons) in the atoms or molecules of the medium. These interactions cause the electromagnetic waves to be absorbed and re-emitted by the particles, resulting in a delay in the overall propagation of the wave. This delay gives the illusion that light is slowing down in the medium.
It's important to note that the speed reduction only applies to the overall propagation of light through the medium, not to individual photons. Photons, which are particles of light, always travel at the speed of light 'c' in a vacuum. However, in a medium, they may experience interactions that cause their path to appear slower when observed at a macroscopic level.
So, light itself is constant, but its speed can be affected by the medium through which it travels due to interactions with the particles in that medium.