The discussion clarifies that while light slows down in water, it returns to its original speed, c, once it exits the medium. The apparent slowing occurs because photons are absorbed and re-emitted multiple times in a medium, affecting their average propagation speed. The refractive index explains this phenomenon, with air having a value of approximately 1.0003 and water around 1.333 for visible light. Thus, light travels faster in air than in water due to these differences in refractive index. Overall, the principles of special relativity maintain that the speed of light in a vacuum remains constant at c.