The statement that the speed of light in a vacuum is constant is one of the fundamental principles in physics and is known as "the constancy of the speed of light." It is a key postulate of Albert Einstein's theory of special relativity. According to this principle, the speed of light in a vacuum, denoted by the symbol 'c,' is the same for all observers, regardless of their relative motion or the motion of the source of light.
However, when light passes through a medium other than a vacuum, such as air, water, or glass, its speed can be different from 'c.' In a medium, light interacts with the atoms or molecules present, leading to a variety of effects, including absorption, scattering, and refraction. These interactions cause the light to slow down and result in a change in its wavelength.
The wavelength of light in a medium can be different from its wavelength in a vacuum due to a phenomenon called dispersion. When light enters a medium, its interaction with the atoms or molecules causes the wave to be delayed and re-emitted. This delay affects different wavelengths of light to varying extents, resulting in a change in the overall wavelength of the light. This change is responsible for the phenomenon of refraction, where light bends as it passes from one medium to another.
Although the wavelength of light changes in a medium, the fundamental principle of the constancy of the speed of light still holds. The speed of light in a medium, denoted by 'v,' is always less than the speed of light in a vacuum ('c'). The relationship between the speed of light in a vacuum, the speed of light in a medium, and the index of refraction of the medium is given by the equation:
v = c/n,
where 'n' represents the refractive index of the medium. The refractive index is a measure of how much the speed of light decreases in a particular medium. So, while the speed of light changes when it passes through a medium, it still obeys the principle that it always travels at the maximum speed possible within that medium.
To summarize, the constancy of the speed of light refers specifically to the speed of light in a vacuum, while the change in wavelength of light in a medium is a consequence of its interaction with the atoms or molecules present. Despite the change in wavelength, the speed of light in the medium remains constant relative to that medium, but it is always less than the speed of light in a vacuum.