When an electromagnetic wave, such as light, travels from one medium to another, its speed can change. This change in speed is due to the interaction of the wave with the atoms or molecules in the medium it is passing through.
In a less dense medium, such as air or vacuum, the atoms or molecules are relatively far apart. When an electromagnetic wave encounters these atoms or molecules, it interacts with them only briefly before moving on to the next one. As a result, the wave can travel relatively freely and at a higher speed.
However, in a denser medium, such as water or glass, the atoms or molecules are more closely packed. When the wave encounters these closely spaced particles, it interacts with them more frequently and for longer durations. This interaction causes the wave to be absorbed and re-emitted by the atoms or molecules, effectively slowing it down.
To understand this phenomenon, it's helpful to consider the wave as a combination of an electric field and a magnetic field oscillating perpendicular to each other. As the wave travels through the medium, the electric field interacts with the charged particles in the atoms or molecules. These charged particles are typically electrons or atomic nuclei.
In a denser medium, the electric field of the wave exerts forces on the charged particles more frequently. The charged particles are accelerated by these forces, which causes them to re-emit the energy they absorbed from the wave. This re-emission process introduces delays and interactions that effectively slow down the wave.
It's important to note that while the speed of light (or the electromagnetic wave) decreases in a denser medium, the frequency remains the same. This relationship is described by Snell's law, which relates the angle and speed of the wave as it undergoes refraction while passing from one medium to another.
In summary, the decrease in speed of an electromagnetic wave in a denser medium is caused by the increased interaction and absorption of the wave's energy by the atoms or molecules in that medium.