* Light travels fastest in a vacuum. This is the speed we typically refer to as "the speed of light" - approximately 299,792,458 meters per second.
* Light slows down when it enters a denser medium. This is because the light interacts with the atoms of the medium, causing it to change direction and take a slightly longer path.
* The amount the light slows down depends on the refractive index of the medium. The higher the refractive index, the slower the light travels.
Here's how it would play out in your scenario:
1. Air: Light travels very close to its speed in a vacuum in air, as air has a refractive index very close to 1.
2. Glass: Glass has a higher refractive index than air, so light slows down as it enters the glass.
3. Water: Water has a higher refractive index than glass, so light slows down further as it enters the water.
Key takeaway: The speed of light decreases as it moves from a medium with a lower refractive index to a medium with a higher refractive index.
