When light or an electromagnetic wave travels through a vacuum, its speed is approximately 299,792,458 meters per second, denoted as "c." This speed is considered constant because it represents the maximum speed at which information or energy can propagate in the universe.
However, when light enters a denser medium, such as glass or water, its speed decreases. This phenomenon occurs due to the interaction of light with the atoms or molecules of the medium.
In a transparent medium, such as glass, the atoms or molecules have an electric charge distribution that interacts with the electric and magnetic fields of the light wave. As the wave passes through the medium, it causes the charged particles in the atoms or molecules to oscillate. These oscillations absorb and re-emit the incoming light waves, which delays the overall transmission of the wave through the medium.
This absorption and re-emission process introduces a time delay and reduces the speed of the wave. The delay occurs because the wave spends some time being absorbed by the medium and then re-emitted in a forward direction. As a result, the effective speed of light in the medium is slower compared to its speed in a vacuum.
The slowing down of light in a denser medium can be quantified using the refractive index (n) of the medium. The refractive index represents the ratio of the speed of light in a vacuum to the speed of light in the medium. Mathematically, it can be expressed as:
n = c/v
Where c is the speed of light in a vacuum and v is the speed of light in the medium. The refractive index is always greater than or equal to 1, indicating that the speed of light in a medium is always slower than its speed in a vacuum.
It's important to note that the frequency of the light wave remains constant when it enters a different medium, but its wavelength changes. This change in wavelength is responsible for the bending or refraction of light at the interface between two media with different refractive indices.