If the speed of light is supposed to be contant, why does it go slower in water than in air?
The speed of light slows down when traveling through a medium such as water because of the interaction between electromagnetic waves and the molecules of the fluid. The exact reason for this effect is not well understood, but it is believed that the water atoms scatter the light waves causing them to travel at different speeds. In contrast, the atoms of air are less likely to interact with the light, so the rays move more quickly through the space. This difference in velocity results in the observed decrease in the speed of light in water compared to air.