Here's why:
* Speed of Light in a Vacuum: The speed of light in a vacuum is a fundamental constant, denoted by 'c', and is approximately 299,792,458 meters per second. This is the fastest anything can travel.
* Interaction with Matter: When light enters a transparent material, it interacts with the atoms and molecules in that material. These interactions cause the light to slow down.
* Refractive Index: The degree to which light slows down in a material is quantified by the material's refractive index. A higher refractive index means the light travels slower through that material. For example, water has a refractive index of about 1.33, meaning light travels about 1.33 times slower in water than in a vacuum.
The formula to calculate the speed of light in a material is:
```
v = c / n
```
where:
* v is the speed of light in the material
* c is the speed of light in a vacuum
* n is the refractive index of the material
Important Note: The speed of light does not change within a material. It slows down when entering the material and speeds up again when exiting.
