Light travels faster in a vacuum than in water, with a speed of approximately 300,000 km/s in a vacuum compared to about 225,000 km/s in water due to interactions with atoms and molecules. This slower speed in water is explained by the absorption and re-emission of photons, which creates delays in the light's passage. Snell's Law can be used to measure the speed of light in different media by calculating the refractive index, which is approximately 1.33 for water. While light's speed appears to decrease in media, the fundamental speed of light remains constant at "c" in a vacuum. Understanding these principles is crucial for grasping concepts in optics and physics.