The constant velocity of electromagnetic (EM) radiation, including light, and the preservation of its wavelength in different media can be explained through the principles of electromagnetism and the behavior of waves.
According to Maxwell's equations, which describe the behavior of electromagnetic fields, the speed of light in a vacuum is determined by two fundamental constants: the permeability of free space (μ₀) and the permittivity of free space (ε₀). This speed, denoted by the symbol "c," is approximately 299,792,458 meters per second.
When light enters a medium, such as glass or water, with a varying refractive index, the interaction of the light with the atoms or molecules in the medium causes the wave to be slowed down. The refractive index of a medium describes how much slower light travels in that medium compared to its speed in a vacuum.
When light passes through a medium, the speed of the wavefront decreases, but the frequency of the light remains constant. The frequency of a wave is the number of wave cycles passing a given point per unit of time and determines the color or energy of the light. Since the speed of light decreases while the frequency remains constant, the wavelength must also change to ensure that the wave equation, c = λν (where c is the speed of light, λ is the wavelength, and ν is the frequency), is satisfied.
However, despite the change in speed and wavelength, the refractive index of a medium does not alter the frequency of light. This phenomenon is described by Snell's law, which states that when light passes from one medium to another, the product of the refractive index and the sine of the angle of incidence is constant. This means that the frequency of the light remains unchanged, and any change in speed is compensated by an equivalent change in wavelength to maintain the constant frequency.
In summary, the constant velocity of light in a vacuum is determined by the fundamental properties of space, and when light passes through a medium with a varying refractive index, the wavelength adjusts to maintain a constant frequency, while the speed of propagation is reduced due to interactions with the medium's atoms or molecules.