The speed of light in a vacuum is approximately 299,792,458 meters per second (or about 186,282 miles per second). This speed is commonly denoted as "c" and is considered to be a fundamental constant of nature.
However, the speed of light can change when it passes through different transparent media, such as air, water, glass, or any other substance. In a medium other than a vacuum, the speed of light is generally slower than its speed in a vacuum.
The speed of light in a medium is determined by the optical properties of that medium, particularly its refractive index. The refractive index is a measure of how much the speed of light is reduced when it travels through a medium compared to its speed in a vacuum.
According to Snell's law, the relationship between the speed of light in a medium (v) and the speed of light in a vacuum (c) is given by:
v = c / n
where "n" is the refractive index of the medium. The refractive index is always greater than or equal to 1, which means that the speed of light in a medium is always less than or equal to its speed in a vacuum.
In summary, while the speed of light in a vacuum is a constant, the speed of light in a medium depends on the refractive index of that medium and is generally slower than its speed in a vacuum.