Water does alter the velocity of light compared to its speed in a vacuum. The speed of light in a vacuum is approximately 299,792,458 meters per second (m/s). When light travels through a medium like water, its speed decreases.
The reduction in speed is due to the interaction of light with the atoms and molecules in the medium. In water, these interactions involve the absorption and re-emission of photons by water molecules, as well as scattering and other phenomena.
The speed of light in water is lower than its speed in a vacuum and is approximately 225,000,000 meters per second (m/s), which is about 75% of its speed in a vacuum.
The specific velocity of light in a particular medium, like water, is determined by its refractive index. The refractive index of water is approximately 1.33, which indicates how much the speed of light is reduced compared to its speed in a vacuum.
It's worth noting that the refractive index of water varies slightly with factors such as temperature and wavelength of light. However, for practical purposes, the value of 1.33 is commonly used to represent the refractive index of water.