Have you ever been curious about the patterns of light created when light shines through a swimming pool? If sunlight is uniform, why does it have such a mottled appearance in the water?

Although light is unique, it is not magical and has a real world physical presence. This presence results in interactions between light and other materials in the world. In the case of water, light and water interact in a variety of ways so as to create the astonishing patterns you see in a swimming pool.

One of these interactions is called refraction. As light travels through a medium, it naturally hits that medium's particles. As a result, light moves more slowly through denser materials. (Note that when we refer to light's speed as constant, we mean within the same medium. Light's speed can vary depending on the medium, though it's always fast!) This slower speed results in a bending of the light toward the normal, a line perpendicular to the boundary of two mediums. The principle is illustrated by the following diagram:

When refraction is combined with the waves of the water, light is concentrated into lines along the pool's bottom. The result is the mottled pattern seen in the example above!