The speed of light in a vacuum, denoted by "c," is a fundamental constant in nature and has a value of approximately 299,792,458 meters per second. When light passes through a medium, such as water or glass, its speed is reduced compared to its speed in a vacuum.
However, it is important to note that the speed of light in a medium depends on the refractive index of that medium. The refractive index is a property of the material and represents how much the speed of light is reduced when it enters the material.
When light travels from one medium to another, such as from air to water, it undergoes a phenomenon known as refraction. Refraction occurs due to a change in the speed of light as it enters a new medium. The change in speed causes the light waves to change direction, and this bending of light is what we observe as refraction.
The wavelength of light does change when it enters a different medium, and this change is responsible for phenomena like dispersion (e.g., when white light is separated into its constituent colors by a prism). However, despite the change in wavelength, the speed of light in the new medium remains constant.
This consistency of the speed of light across different media can be explained by the interaction between the electric and magnetic fields of light and the charged particles (usually electrons) within the material. When light passes through a medium, the electric and magnetic fields induce oscillations in the charged particles, causing them to emit secondary waves. These secondary waves interfere with the original wave and determine the speed at which the overall wave propagates through the medium.
In a sense, the speed of light in a medium is an average velocity that takes into account the interactions between the electromagnetic wave and the charged particles in that medium. The wavelength may change, but the speed remains the same for a given medium.
It is worth noting that the speed of light in a medium is still significantly slower than its speed in a vacuum. This reduction in speed is responsible for effects such as the bending of light, which allows us to observe phenomena like the apparent displacement of objects in water or the formation of rainbows.