The speed of light slows down when it travels through a medium (like water, glass, or air) compared to its speed in a vacuum, which is about 299,792 kilometers per second. Here’s why and how it differs:
- In a vacuum, light travels at its maximum speed because there’s nothing to interact with—no particles to slow it down.
- In a medium, light interacts with the atoms or molecules present. It gets absorbed and re-emitted many times as it passes through the material. This process doesn’t change the frequency of light but delays its overall travel, making it appear slower.
- The denser the medium, the more these interactions occur, and the more the light slows down. For example:
- In air, light slows only slightly.
- In water, it travels at about 75% of its vacuum speed.
- In glass, it’s even slower—around 65% of its vacuum speed.
- The amount light slows down in a medium is described by the refractive index. A higher refractive index means slower light.
This slowing down leads to important phenomena like refraction (bending of light), which is why a straw looks bent in a glass of water.