When an electromagnetic wave, such as light, travels from one medium to another, its speed can change. The change in speed is primarily attributed to the wave interacting with the atoms or molecules in the new medium.
In a less dense medium, such as air or vacuum, the atoms or molecules are relatively far apart. When an electromagnetic wave passes through this medium, it encounters minimal obstruction or interaction with the atoms or molecules. As a result, the wave can travel relatively faster.
However, in a denser medium, such as water or glass, the atoms or molecules are closer together. When an electromagnetic wave enters this medium, it interacts more frequently with the atoms or molecules. These interactions cause delays as the wave gets absorbed and re-emitted by the particles. This absorption and re-emission process takes some time and slows down the wave's overall speed.
To understand this concept, consider the analogy of a group of people walking through an empty field and then entering a crowded marketplace. In the field, where people are spread out, they can move quickly. But in the crowded marketplace, they encounter more obstacles, need to navigate around others, and consequently move slower.
In summary, the decrease in speed of an electromagnetic wave in a denser medium is due to the increased interaction and absorption of the wave by the atoms or molecules in that medium.