Attenuation of Light
The light that humans see is just a small part of the total electromagnetic radiation that is received from the sun. We see what is called the visible spectrum. The actual colors within the visible spectrum are determined by the wavelengths of the light: the longer wavelengths are red and orange; the shorter wavelengths are green, blue, and violet. Many fish, however, can see colors that we do not, including ultraviolet.
When light enters water, its intensity quickly decreases and its color changes. These changes are called attenuation. Attenuation is the result of two processes: scattering and absorption. The scattering of light is caused by particles or other small objects suspended in the water — the more the particles, the more the scattering. The scattering of light in water is somewhat similar to the effect of smoke or fog in the atmosphere. Coastal waters generally have more suspended material due to river input, material stirred up from the bottom, and increased plankton. Because of this greater amount of suspended material, light usually penetrates to a lesser depth. In relatively clear offshore water, light penetrates to a greater depth.
Light absorption is caused by several things, such as the light being converted into heat or used in chemical reactions such as photosynthesis. The most important aspect for fishing is the influence of the water itself on the absorption of light. The amount of absorption is different for different wavelengths of light; in other words, various colors are absorbed differently. The longer wavelengths, such as red and orange, are absorbed very quickly and penetrate into the water to a much shallower depth than the shorter blue and violet wavelengths.
Absorption also restricts how far light penetrates into the water. At about three meters (about 10 feet), roughly 60 percent of the total light (sunlight or moonlight) and almost all the red light will be absorbed. At 10 meters (about 33 feet), about 85 percent of the total light and all the red, orange, and yellow light have been absorbed. This has a direct bearing on how a fish perceives a fly. At a depth of 10 feet, a red fly appears gray, and it eventually appears black as the depth increases. With the increasing depth, the now dimming light becomes bluish and eventually black when all the other colors are absorbed.
The absorption or filtering out of color also works in a horizontal direction. So again, a red fly that is only a few feet from a fish appears gray. Similarly, other colors also change with distance. For a color to be seen, it must be hit by light of the same color and then reflected in the direction of the fish. If the water has already attenuated or filtered out) a color, that color will appear gray or black. (Fluorescent colors, which I will come to shortly, behave a little differently.)
It should now be clear how the depth of the water or distance from a fish affects the visibility of your fly. In extremely shallow and very clear water, colors may look similar to their appearance in the air; as your fly gets just three feet deep or three feet away from a fish — or less if the water has limited clarity — the colors will start to change, often with surprising results.