In our current understanding of physics, light always travels at the same speed in a vacuum, which is approximately 299,792,458 meters per second (often rounded to 3.0 x 10^8 m/s). This speed is denoted by the symbol "c" and is considered a fundamental constant of nature.
However, it is possible for light to travel slower than its original velocity when passing through a medium other than a vacuum, such as a transparent material like water, glass, or air. When light enters a medium, it interacts with the atoms or molecules of the material, which can affect its speed. The reduction in speed is due to the absorption and re-emission of light by the particles in the medium.
The speed of light in a medium is described by a parameter called the refractive index (n) of that medium. The refractive index indicates how much slower light travels in the medium compared to its speed in a vacuum. The refractive index is always greater than or equal to 1, with a value of 1 indicating that light travels at its maximum speed in a vacuum.
The relationship between the speed of light in a vacuum (c) and its speed in a medium (v) can be expressed by the equation:
v = c / n
where n is the refractive index of the medium.
When light travels through a medium where the refractive index is greater than 1, it slows down. The degree to which light slows down depends on the specific material and its refractive index. For example, in water, the refractive index is approximately 1.33, so light travels at around 225,000,000 meters per second (or 2.25 x 10^8 m/s) when passing through it.
As for the change in brightness, it is important to note that the speed of light itself does not directly affect its brightness. The brightness or intensity of light is determined by the number of photons (individual packets of light energy) per unit area. The speed of light affects how quickly photons reach a particular point, but not their quantity.
However, when light passes through a medium, it can undergo various interactions, such as absorption and scattering, which can alter its intensity. These interactions can cause some of the photons to be absorbed by the medium, resulting in a decrease in brightness as the light travels through it. The degree of absorption and scattering depends on the properties of the medium and the wavelength of the light.
In summary, when light travels slower than its original velocity in a medium, it is due to the interaction of light with the atoms or molecules in that medium. The speed of light in a medium is described by the refractive index, and the degree to which it slows down depends on the specific material. However, the change in brightness is not directly related to the speed of light but can be influenced by interactions like absorption and scattering that occur as light passes through a medium.