The speed of light changes when it travels through different mediums due to a property called refractive index. In a vacuum, light travels at its maximum speed—about 299,792 kilometers per second (km/s). However, when it enters a medium like water or glass, it slows down.
Here’s how it works:
- In water: Light travels at about 225,000 km/s. Water has a refractive index of approximately 1.33, meaning light is slowed down compared to its speed in a vacuum.
- In glass: Light travels even slower, around 200,000 km/s, depending on the type of glass. Glass typically has a refractive index of about 1.5.
The denser the medium (optically), the slower the light travels. This happens because the light interacts with the particles in the medium, getting absorbed and re-emitted, which delays its overall progress. This slowing down also causes the light to bend or refract, changing direction as it enters the new medium.