The speed of light in a medium other than vacuum is slower because of the interaction between light and the atoms or molecules of the medium. In a vacuum, light travels at its maximum speed, which is approximately 299,792,458 meters per second (often rounded to 300,000 kilometers per second). This speed is commonly denoted as the speed of light in a vacuum, symbolized by the letter 'c'.
When light travels through a medium, such as air, water, glass, or any other transparent substance, it encounters atoms or molecules that make up the material. These atoms or molecules have electric charges associated with them, which respond to the oscillating electric and magnetic fields of the light wave.
As the light wave interacts with these charged particles, it induces a polarization effect in the atoms or molecules. The charged particles temporarily reconfigure themselves in response to the electric and magnetic fields of the light wave passing through them. This reconfiguration takes a finite amount of time.
This process of polarization and reconfiguration slows down the propagation of the light wave. Essentially, the electric and magnetic fields of the light wave cause a delay as they interact with the charged particles in the medium. The light wave has to continuously interact with and transfer energy to these particles, which leads to a reduction in its speed.
The speed of light in a medium is typically denoted as 'v', and it is given by v = c/n, where 'n' is the refractive index of the medium. The refractive index represents how much slower light travels in the medium compared to its speed in a vacuum. It is a property specific to each medium and depends on the density and composition of the material.
In summary, light slows down in mediums other than vacuum due to the interaction between the light wave and the charged particles (atoms or molecules) in the medium, which leads to a delay in the propagation of the light wave.