Here's why:
* Light interacts with matter: Light isn't just traveling through empty space. When it enters a medium like water or glass, it interacts with the atoms and molecules in that material. These interactions slow down the light's propagation.
* Frequency stays constant: The frequency of light, which determines its color, remains constant during refraction.
* Wavelength changes: Since the speed of light changes, and frequency remains the same, the wavelength of light must change to accommodate this. The wavelength becomes shorter in denser mediums.
Important Note: While the speed of light *within* the medium decreases, the speed of light *in a vacuum* is always constant and is considered the ultimate speed limit in the universe.
