The statement that the speed of light in a vacuum is constant is a fundamental principle of physics and is known as the postulate of the constancy of the speed of light. It is one of the key insights of special relativity, which has been extensively tested and confirmed by numerous experiments.
In special relativity, the speed of light in a vacuum, denoted by "c," is considered to be a fundamental constant of nature. It is the maximum speed at which any information or causal influence can travel. This means that the speed of light in a vacuum is the same for all observers, regardless of their relative motion.
When light travels through different media, such as air, water, or glass, its speed changes due to interactions with the atoms or molecules of the material. This change in speed is due to the photons of light being absorbed and re-emitted by the particles in the medium, which causes a delay in their overall progress.
However, it's important to note that the constancy of the speed of light refers specifically to its value in a vacuum. The change in the speed of light in different media is due to the interaction of light with matter, which affects its propagation but does not alter its fundamental nature as a constant in a vacuum.
So, while the wavelength and frequency of light can be affected by the medium through which it passes, the speed of light itself remains constant in a vacuum, as established by the fundamental principles of special relativity.