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into the liquid medium of our eye is not enough to allow us to focus our vision, and everything appears out of focus and blurred. The diving mask provides the necessary air to eye interface for clear vision. It also creates a water to air interface at the front of the mask, and light is first refracted as it enters the mask. The visual result is magnification as we see the object as though the light had traveled in a straight line. Objects viewed through your mask appear larger, by about four-thirds, and closer, by about one quarter, than they really are. This is a real optical difference, not an illusion. In underwater photography, a camera must be focused on the apparent distance rather than the measured distance. Refraction causes a straight stick to appear larger and bent when it is partly immersed in water; it causes your feet to appear much closer when you look down at them as you stand in a pool. Absorption refers to the changes in the color and intensity of light as it passes through the water. The different wavelengths of light possess different energy. Red has the longest wavelength and the least energy; shorter wavelengths have progressively more energy. As light moves through the water, the molecules of water absorb the energy of the light. Visually, the light is filtered out as it passes through the water column. The least energetic wavelengths are absorbed first, then each in turn to the most energetic as the light travels an increasing distance through the water. Reds may disappear even in shallow water. As the depth increases further colors in the spectrum are absorbed, and soon bluish colors are all that remain. At greater depths even the blues are filtered out, and one enters the “world without light” of the deep ocean. A dive light or a camera strobe provides close-up illumination and allows us to “restore” colors underwater. Diffusion is the scattering of light as it interacts with the molecules in the medium it is passing through. Light is scattered in random directions as it strikes particulates suspended in the water. A similar diffusion occurs when light passes through fog and interacts with the water droplets suspended in the air. Light penetrating into the water decreases with depth due to both diffusion and absorption. If the water is very turbid (filled with suspended particles), diffusion and absorption occur rapidly, and the diver sees less, both because there is less light and because the light that is available is scattered quickly. Even in the clearest water, 50 meters/165 feet of visibility is exceptional. Reflection occurs when light waves bounce off a surface. If the surface is smooth and polished, reflection is as in a mirror. When light strikes the surface of water, some of the light enters the water and some of the light is reflected. If the light enters vertically as when the sun is directly overhead, most of the light enters the water, but as the angle approaches horizontal (as the sun nears the horizon) more of the light striking the surface will be reflected. Near sunrise or sunset the amount of light entering the water and providing illumination for a diver can be significantly reduced. The same reflection occurs when a dive light is shined at the surface from underwater. Especially if the surface is calm, you can see the beam of your dive light reflected back into the water from the surface at night. As the angle becomes more pronounced, more light is reflected. NAUI Master Scuba Diver 98 Diving Physics Red Orange Yellow Green Violet Indigo Blue Surface Depth 3m(10) 6m(20) 9m(30) 12m(40) 15m(50) FIGURE 3-13. COLOR LOSS WITH DEPTH


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