The speed of light in a vacuum is approximately 299,792,458 meters per second, often rounded to 300,000 kilometers per second. This value is commonly denoted by the symbol "c" in scientific equations.
However, when light passes through a medium other than a vacuum, such as air, water, or glass, its speed can be slower than its speed in a vacuum. The speed of light in a medium is determined by the optical properties of that medium, specifically its refractive index. The refractive index is a measure of how much the speed of light is reduced when it travels through a particular material compared to its speed in a vacuum.
The relationship between the speed of light in a vacuum (c) and the speed of light in a medium (v) is described by Snell's law in classical physics and Fermat's principle in optics. According to these principles, the speed of light in a medium is given by v = c/n, where n is the refractive index of the medium.
Different materials have different refractive indices, which can vary with the wavelength of light. This is why light appears to bend or change direction when it travels from one medium to another, such as when light passes from air to water or from air to a glass prism.
In summary, the speed of light in a vacuum is constant and approximately 299,792,458 meters per second. However, the speed of light can be slower when it passes through a medium, and this reduction in speed is determined by the refractive index of the medium.