* Refractive Index: The speed of light in a medium is determined by its refractive index. The refractive index measures how much the speed of light is slowed down in that medium compared to a vacuum.

* Air: Air has a refractive index very close to 1. This means light travels almost as fast in air as it does in a vacuum.

* Glass: Glass has a refractive index greater than 1 (typically around 1.5). This means light travels slower in glass than in a vacuum or air.

The reason for the slower speed:

Light interacts with the atoms of the medium it travels through. In denser materials like glass, there are more atoms packed together. This interaction with the atoms causes the light to slow down.

Think of it like this: Imagine light as a car driving through a crowded city vs. driving on an open highway. The car will move slower in the city because it has to navigate around obstacles (atoms).