Light bends when it passes through water because of a phenomenon called refraction. Refraction is the bending of light as it travels from one medium to another. In this case, the light is traveling from air, which is a less dense medium, to water, which is a denser medium.

When light enters a denser medium, it slows down. This is because the molecules in the denser medium are closer together, so they can interact with the light more easily. As the light slows down, it bends towards the normal. The normal is an imaginary line that is perpendicular to the surface at the point where the light enters the water.

The amount of bending depends on the difference in the refractive indices of the two media. The refractive index of a medium is a measure of how much light bends when it enters the medium. The higher the refractive index, the more the light bends.

The refractive index of water is about 1.33, while the refractive index of air is about 1.00. This means that light bends about 33% when it enters water from air.

The bending of light can be seen in many everyday situations. For example, when you look at a fish in an aquarium, the fish appears to be closer to the surface than it actually is. This is because the light from the fish is bent when it enters the water, making it appear to come from a different point.

Refraction is also used in many optical devices, such as lenses and prisms. Lenses are used to focus light, and prisms are used to split light into its component colors.