The velocity of electromagnetic (EM) radiation, including light, is constant because it is determined by fundamental properties of space and time. In a vacuum, EM radiation travels at a constant speed known as the speed of light, denoted by 'c' in scientific equations. The value of 'c' is approximately 299,792,458 meters per second.
This constancy of the speed of light is a fundamental principle of physics known as the postulate of special relativity, proposed by Albert Einstein. According to this principle, the speed of light in a vacuum is the same for all observers, regardless of their relative motion. It is a universal constant and serves as an upper limit for the speed at which information or signals can be transmitted through the universe.
When light travels through a medium with a varying refractive index, such as a transparent substance like glass or water, its velocity decreases compared to its speed in a vacuum. This decrease in velocity occurs because light interacts with the atoms or molecules of the medium, leading to absorption and re-emission processes that cause a delay in the propagation of the light wave.
However, while the velocity of light changes in a medium, its frequency and wavelength remain unchanged. The refractive index of a medium determines how much the velocity of light is reduced, but it does not affect the frequency or wavelength of the light wave. This behavior is described by Snell's law, which relates the angle of incidence and refraction as light passes through the boundary between two different media with different refractive indices.
According to Snell's law, as light enters a medium with a higher refractive index, it bends towards the normal (a line perpendicular to the surface), causing a change in the direction of the light wave. This bending of the light wave is what gives rise to phenomena such as refraction and the formation of rainbows.
In summary, the constancy of the velocity of light is a fundamental property of the universe, while the change in velocity and direction of light when passing through a medium is a consequence of its interaction with the atoms or molecules of that medium. However, the frequency and wavelength of light remain unchanged during this process.