The speed of light in a medium, such as water or glass, is indeed different from the speed of light in a vacuum. When light passes through a medium, it interacts with the atoms or molecules of that medium, causing a delay in its propagation. This delay is a result of the absorption and re-emission of photons by the atoms or molecules.
The speed of light in a medium is given by the equation:
v = c/n
where v is the speed of light in the medium, c is the speed of light in a vacuum, and n is the refractive index of the medium.
The refractive index is a property of the medium and is related to the speed of light in that medium. It is defined as the ratio of the speed of light in a vacuum to the speed of light in the medium. Each medium has its own refractive index, which determines how much the speed of light is reduced when it enters that medium.
While the speed of light is different in different media, the fundamental nature of light itself remains the same. The speed of light in a vacuum, denoted by "c," is considered a fundamental constant of nature. It represents the maximum speed at which information or energy can be transmitted through space.
Although the speed of light is different in different media, the relationship between the speed of light and its wavelength remains consistent. When light enters a medium, its wavelength changes due to the interaction with the atoms or molecules. This change in wavelength is responsible for phenomena like refraction and dispersion. However, the speed at which the modified light wave travels through the medium is determined by the refractive index and remains consistent within that medium.
In summary, while the speed of light in different media is not the same, the fundamental constant speed of light in a vacuum remains unchanged. The refractive index of a medium affects the speed of light within that medium, but it does not alter the fundamental nature of light itself.