Why light travels more slowly in water
Light is an electromagnetic wave: its electric field pushes on charged particles. As the wave enters water, it shifts the electrons in the molecules slightly back and forth. Those moving charges produce their own electromagnetic wave, which combines with the incoming one. The combined wave advances through the water with a delay compared with light traveling through a vacuum.
This is a collective effect of the water, not a sequence of tiny collisions that make light ricochet between molecules. The molecules respond and pass the disturbance along, much as a crowd can transmit a wave even though each person moves only a little.
The speed changes; the color does not
The ratio between light’s vacuum speed and its speed in a material is called the refractive index. Water’s refractive index is about 1.33 for visible light, so the wave’s crests are closer together there than in air. At the boundary, the wave’s frequency—the rate at which crests arrive—stays fixed. With less distance between crests per cycle, the wave’s speed is lower.
That spacing change does not, by itself, change the light’s color. Color corresponds to frequency, which remains the same as light crosses into or out of the water. The spacing returns to its former value when the wave leaves.
The change in speed also explains why a beam bends when it enters water at an angle: one side of the wave reaches the slower medium first, so the wave turns. That bending is why a submerged object can appear shifted from where it really is.
