When an electromagnetic wave travels from one medium to another with a different refractive index (density), its speed changes. This phenomenon is described by Snell's law, which governs the refraction of light.
The speed of light in a vacuum is constant and denoted by 'c.' However, when light passes through a medium, such as air, water, or glass, its speed decreases. This is because the interaction of light with the atoms or molecules in the medium affects its propagation.
In a denser medium, the atoms or molecules are typically more closely packed together. When an electromagnetic wave enters this medium, the electric and magnetic fields of the wave interact with the charged particles in the material. As a result, the wave is absorbed and re-emitted by the particles, causing a delay in its transmission.
This absorption and re-emission process leads to an effective slowdown of the wave, reducing its speed. The denser the medium, the more interactions occur, and the greater the decrease in speed.
The change in speed of light when it enters a new medium is described by the refractive index (n) of that medium. The refractive index is the ratio of the speed of light in a vacuum to the speed of light in the medium (v):
n = c/v
Therefore, when light travels through a denser medium with a higher refractive index, the speed of light decreases due to the increased interactions with the medium's particles.