Light travels slower in water due to its interaction with water molecules, which absorb and re-emit photons, creating a delay in propagation. The phase velocity of light in a medium is determined by the medium's permittivity and permeability, as described by Maxwell's equations. While light in a vacuum travels at speed c, in a medium like water, it travels at c/n, where n is the refractive index. The discussion also highlights the distinction between electromagnetic waves and light, clarifying that not all electromagnetic waves are classified as light. Understanding these concepts requires a grasp of the underlying mathematics and physics, particularly regarding wave behavior and interactions at material boundaries.