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. That is, if one were to
take 7 flashlights, each of which was giving off one of the listed colors,
and shine all the different colors on a white wall, the spot of light would
be white! A white light, therefore, gives off all the colors.
Water acts as a selective filter. If one were to suspend
a white light above the surface of a tank of water that was 1000' deep, the
colors from the white light would be filtered out selectively one-by-one.
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, etc. 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. So, 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, 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 lose 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.
Neon colors do not loose their color like spectrum colors
do. This author has video photographed a red stripe on a wet suit turn to
black as a diver descended. Neon red and "hot pink" still were sending out
bright color at 100'. That is because they fluoresce. Ultraviolet is found
after violet on the spectrum. It is invisible to humans. It, like violet,
goes to extreme depths. When a neon color is struck by the invisible ultraviolet
it glows.
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
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 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. There are several pollutants
that decrease water's visibility and also change the color and color penetration.
If the water has silt and/or clay in it, 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 a runoff bringing
clay into the lake. I have seen the visibility close to shore drop from 30
feet to 1 foot in several minutes! If the 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, and when the summer
visitors arrive the amount of human waste runoff increases promoting an algae
bloom. There are other pollutants that color the water. Tannic acid, especially
from pine trees, can color the water brown even though it may be clear. Rotting
vegetation and iron may cause a brown color, and paper mill runoff can leave
it looking similar to milk.
