The speed of light in a medium is influenced by its interaction with the atoms or molecules in that medium. When light passes through a medium, it can interact with the particles within it, causing delays in its propagation and resulting in a slower overall speed compared to its speed in a vacuum.
This phenomenon is primarily due to the electromagnetic interactions between the light waves and the charged particles in the medium. When an electromagnetic wave, such as light, encounters a charged particle, it induces oscillations in the particles' electric fields. These induced oscillations, in turn, create secondary electromagnetic waves that interfere with the original wave.
In a denser medium, there is a higher concentration of particles per unit volume, which means there are more opportunities for the light waves to interact with these particles. As a result, the interference and reemission processes occur more frequently, causing the overall speed of light to decrease.
Another way to understand this is through the concept of refractive index. The refractive index of a medium quantifies how much slower light travels in that medium compared to its speed in a vacuum. It is defined as the ratio of the speed of light in a vacuum to the speed of light in the medium. The refractive index depends on the properties of the medium, such as its density, atomic or molecular structure, and other factors.
In summary, light travels slower in denser mediums because the presence of more particles increases the frequency of interactions between light and those particles, leading to delays in the propagation of the light waves.