Here's why:
* The speed of light in a vacuum is a constant: This is denoted by 'c', approximately 299,792,458 meters per second. It's the fastest speed anything can travel in the universe.
* Light slows down in other mediums: When light passes from a vacuum into a medium like air, water, or glass, it interacts with the atoms of that medium. This interaction causes the light to slow down.
* The index of refraction: The amount that light slows down in a medium is determined by its refractive index. The higher the refractive index, the more the light slows down. For example, water has a refractive index of 1.33, meaning light travels about 1.33 times slower in water than in a vacuum.
In summary:
* Vacuum: Light travels at its maximum speed (c).
* Other mediums: Light travels slower than c, depending on the medium's refractive index.
