When light travels through a medium, such as a transparent material like glass or water, it can be slowed down due to interactions between the photons (particles of light) and the atoms or molecules of the medium. This phenomenon is known as "slowing down" or "reduction in speed" of light in a medium. The process can be better understood by considering the interaction of light with atoms or molecules.
In a transparent medium, such as glass, the atoms or molecules have an electron cloud surrounding their nuclei. When light passes through the medium, the electromagnetic waves associated with light can interact with the electrons in the atoms or molecules. This interaction can lead to the absorption and subsequent re-emission of photons by the electrons. The absorbed energy causes the electrons to briefly enter an excited state.
The re-emission of the absorbed photons by the electrons introduces a time delay between the absorption and re-emission processes. This time delay effectively slows down the overall speed of light as it travels through the medium. The collective effect of these interactions between photons and atoms or molecules results in a reduced speed of light propagation.
It's important to note that while some photons may pass through the medium unaffected, the majority of photons interact with the atoms or molecules, leading to an overall slowdown of the light. The extent of slowing down depends on various factors, including the nature of the medium, the density of atoms or molecules, and the frequency of the light.
This phenomenon is described by the refractive index of the medium, which quantifies the degree to which light is slowed down when passing through it. The refractive index is a property of the material and is typically greater than 1, indicating that light travels slower in the medium compared to its speed in a vacuum.
So, while some photons may not interact with the atoms or molecules and pass through unaffected, the majority of photons do interact and contribute to the overall reduction in the speed of light as it travels through a medium.