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The light spectrum is well known. "ROY G. BIV" is an acronym used to remember the colors from one end to the other. From left to right the letters stand for: Red, Orange, Yellow, Green, Blue, Indigo, and Violet. A mixture of all the colors makes white light. If one were to take 7 flashlights, each of which was giving off one of the listed colors, and shine all the different lights on a white wall the spot of light would be white. A white light is composed of all the colors. There are 3 primary colors: Red, blue, and green. If those colors are combined the spot would also be white as is depicted in the picture below.
Water acts as a selective filter. If a white light is suspended above a tank of water that was 1000' deep the colors from the white light would be filtered out selectively (one-by-one) as one descends. It is gradual. There is no abrupt interface. For example, most of the red is gone from the light after 10 feet. Some of the orange is gone. Less of the yellow is lost. At 25' most of the orange is gone. At 35' most of the yellow is gone. This continues through the spectrum until all that is left is violet light and that fades out after hundreds of feet. At the bottom of this 1000' tank of water there would be little or no light.
Selective filtration creates conditions that make diving interesting. If a diver is bleeding at 60', where there is no red light, the diver bleeds a greenish-black blood. Taking a photograph at 30' would result in most objects appearing green, blue, violet, and/or black. Taking the same photograph with a flash (white light) would reveal startling colors that were not seen by the diver. Remember, the selective filtering by water occurs in any direction. So a camera's flash will also loose most of its true color effectiveness after a distance of as little as five feet. That is because the light would leave the camera, hit the object 5 feet away and then return to the camera. In that 10 foot travel distance most of the red would be gone and the orange would be diminished.
The above photograph was taken at a depth of about 50'. The rag in the center is a bright red but at this depth there is little red reflected.
This is the same red rag taken in the same spot with a flash. Notice also the sponges and coral show more color.
The above 2 photos were taken at the same spot in Palau in November 2015. The left is without the video light and the right is with the light. The video light has the characteristics of the sun.
Neon colors do not loose their color like spectrum colors do. This author has video photographed a red stripe on the side of a wet suit turn to black as a diver descended. The stripe could not be seen. On another part of the suit the neon red and "hot pink" still were sending out their bright colors at 100'. That is because they fluoresce. Ultraviolet is found after violet on the spectrum. It is invisible to humans. Like violet it goes to extreme depths. When a neon color is struck by the invisible ultraviolet it glows or fluoresces.
Notice how the neon pink is still fluoresces at depth where the red is gone but the ultraviolet is not.
At the other end of the spectrum is the red light that is filtered out by water rapidly. Beyond the red light there is a part of the spectrum that humans cannot see but can feel. It is infrared. (In the far red.) It is also known as heat. That is the energy that one can feel standing in front of a fireplace even though all the hot air, smoke and gases are going up the chimney. Heat energy travels at the speed of light. You thought red light had trouble going deep in water. Infrared cannot penetrate even 1mm. So you might wonder how the oceans get warm if heat cannot get down into the water? It's not the heat from the sun that makes the oceans warm. It is all the colors that are selectively filtered by the water. They are "captured" by the molecules of water and are converted to heat energy because the molecules are made to move faster.
Carry a flashlight or video light with you when you are scuba diving. Even a small light will reveal colors that are startling and would have gone unnoticed.
The above information assumes the water has good visibility. Patty Ryan in the Sept./Oct. issue of Sportdiver, states that the theoretical visibility for distilled water is 242 feet. Seeing further than that underwater is not possible. In open water there are several pollutants that decrease water's visibility and also change the color and color penetration. If the water contains silt and/or clay the visibility may drop to almost zero and will have a brownish color. There are times when a lake, such as Minnewaska in New York State, will have a beautiful blue water color with a visibility of 35 to 40 feet. A sudden thunderstorm may cause runoff that will carry clay into the lake. I have seen the visibility close to shore drop from 30 feet to 1 foot in several minutes. If water has an excessive growth of algae the color of the water will be green. The more the algae the darker the green and the less the visibility. Sometimes the growth is seasonal. Some bodies of water have great visibility in the winter, especially under ice where the affect of the wind stops and the bottom sediment is not stirred up. In the summer the extra sunlight and waste from summer visitors promotes algae blooms. There are other pollutants that color the water. Tannic acid, especially from pine trees, can color the water brown even though it may remain clear. Rotting vegetation and iron may cause a brown color and paper mill runoff can leave it looking similar to milk.
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