Here's why:

* Light travels at different speeds in different mediums. This is because the speed of light is determined by the refractive index of the material it's traveling through.

* Glass has a higher refractive index than air. This means light slows down as it moves from air into glass.

* The speed of light in a vacuum is the fastest it can travel. We call this "c" and it's approximately 299,792,458 meters per second.