How does the speed of light change when entering a denser medium?

When light travels from a vacuum or a less dense medium into a denser medium, such as water or glass, it slows down. This decrease in speed is a result of the interaction between light waves and the particles in the denser medium.

In a vacuum, light travels at its maximum speed, approximately 299,792 kilometers per second. However, when light enters a denser material, it encounters more particles, which affects its propagation speed. The increased number of interactions with these particles causes the light waves to take longer to pass through the medium, effectively reducing their speed.

This phenomenon is further explained by the concept of refractive index, which is a measure of how much light slows down in a particular medium compared to its speed in a vacuum. A higher refractive index indicates a greater reduction in light speed. Therefore, the correct understanding of how light behaves when moving into a denser medium helps clarify why its speed decreases.