Visibility under water depends on the amount of light reaching the particular depth. Illumination itself depends on the thickness of the layer, on the reflection and scatter of light rays in the water medium. 18% of them reach a depth of 18m and some 1% reach as deep as 100m. Besides this fact, a part of the coming from the sun is reflected by the surface of the water. The amount of reflected light depends on the angle between the rays and the surface of the water, also, on the quantity of air bubbles in the surface layer that have been formed by the motion of the water. 

The most important reason for the low visibility under water is the weakening of the refraction capability of human eyes. In the open air, it is sufficient because the refraction quotient of light is 1 and hat of the human optical system is 1.38. Water has a refraction quotient of 1.33, that is, close to that of the eyes. In this case, light is only slightly refracted and the image is formed far beyond the retina, corresponding to long-sightedness of 20 diopters. The function of the mask is to provide normal conditions for underwater visibility by introducing a layer of air with a quotient of 1. Contact lenses can be used instead of a mask. They must have, however, a refraction quotient greater than 1.4 to ensure normal vision under water.

To function effectively under water, divers must understand the changes that occur in their visual perception under water. Many of these changes are caused simply by the fact that light, the stimulus for vision, travels through water rather than air; consequently it is refracted, absorbed, and scattered differently than in air. Refraction, absorption, and scatter all follow physical laws and their effects on light can be predicted; this changed physical stimulus can in turn have pronounced effects on our perception of the underwater world.

Refraction
In refraction, the light rays are bent as they pass from one medium to another of different density. In diving, the refraction occurs at the interface between the air in the diver's mask and the water. The refracted image of an underwater object is magnified, appears larger than the real image, and seems to be positioned at a point three-fourths of the actual distance between the object and the diver's faceplate. 

This displacement of the optical image might be expected to cause objects to appear closer to the diver than they actually are and, under some conditions, objects do indeed appear to be located at a point three- fourths of their actual distance from the diver. This distortion interferes with hand-eye coordination and accounts for the difficulty often experienced by novice divers attempting to grasp objects under water. At greater distances, however, this phenomenon may reverse itself, with distant objects appearing farther away than they actually are. The clarity of the water has a profound influence on judgments of depth: the more turbid the water, the shorter the distance at which the reversal from underestimation to overestimation occurs. For example, in highly turbid water, the distance of objects at 3 or 4 feet (0.9 or 1.2 m) may be overestimated; in moderately turbid water, the change might occur at 20 to 25 feet (6.1 to 7.6 m); and in very clear water, objects as far away as 50 to 75 feet (15.2 to 22.9 m) might be underestimated.

It is important for the diver to realize that judgments of depth and distance are probably inaccurate. As a rough rule of thumb, the closer the object, the more likely it will appear too close, and the more turbid the water, the greater the tendency to see it as too far away. Training to overcome inaccurate distance judgments can be effective, but it is important that it be carried out in water similar to that of the proposed dive or in a variety of different types of water. In addition, training must be repeated periodically to be effective.

Changes in the optical image result in a number of other distortions in visual perception. Mistakes in estimates of size and shape occur. In general, objects under water appear to be larger by about 33 percent than they actually are. This often is a cause of disappointment to sport divers, who find, after bringing catches to the surface, that they are smaller than they appeared under water. Since refraction effects are greater for objects off to the side of the field of view, distortion in the perceived shape of objects is frequent. Similarly, the perception of speed can be influenced by these distortions; if an object appears to cross the field of view, its speed will be increased because of the greater apparent distance it travels.

These errors in visual perception and misinterpretations of size, distance, shape, and speed caused by refraction can be overcome, to some extent, with experience and training. In general, experienced divers make fewer errors in judging the underwater world than do novice divers. However, almost all divers are influenced to some extent by the optical image, and attempts to train them to respond more accurately have met with some, but not complete, success.

Although the refraction that occurs between the water and the air in the diver's face mask produces these undesirable effects, air itself is essential for vision. For example, if the face mask is lost, the diver's eyes are immersed in water, which has about the same refrac- tive index as the eyes. Consequently, no normal focus- ing of light occurs and the diver's vision is impaired immensely. The major deterioration is in visual acuity; other visual functions such as the perception of size and distance are not degraded as long as the object can be seen. The loss of acuity, however, is dramatic, and acuity may fall to a level that would be classified as legally blind (generally 20/200) on the surface. While myopes (near-sighted individuals) do not suffer quite as much loss in acuity if their face masks are lost as individuals with 20/20 vision do, the average acuities of the two groups, myopes and normals, were found to be 20/2372 and 20/4396, respectively, in one study of underwater acuity without a mask.

Scatter
Scatter occurs when individual photons of light are deflected or diverted when they encounter suspended particles in the water. Although scattering also occurs in air, it is of much greater concern under water because light is diffused and scattered by the water molecules themselves, by all kinds of particulate matter held in suspension in the water, and by transparent biological organisms. Normally, scatter interferes with vision and underwater photography because it reduces the contrast between the object and its background. This loss of contrast is the major reason why vision is so much more restricted in water than in air (Duntley 1963, Jerlov 1976); it also accounts for the fact that even large objects can be invisible at short viewing distances. In addition, acuity or perception of small details is generally much poorer in water than in air, despite the fact that the optical image of an object under water is magnified by refraction (Baddeley 1968). The deterioration increases greatly with the distance the light travels through the water, largely because the image-forming light is further interfered with as it passes through the nearly transparent bodies of the biomass, which is composed of organisms ranging from bacteria to jellyfish.

Absorption
Light is absorbed as it passes through the water, and much of it is lost in the process. In addition, the spectral components of light, the wavelengths that give rise to our perception of color, are differentially absorbed. Transmission of light through air does not appreciably change its spectral composition, but transmitting light through water, even through the clearest water, does, and this can change the resulting color appearance beyond recognition. In clearest water, long wavelength or red light is lost first, being absorbed at relatively shallow depths. Orange is filtered out next, followed by yellow, green, and then blue. Other waters, particularly coastal waters, contain silt, decomposing plant and animal material, and plankton and a variety of possible pollutants, which add their specific absorptions to that of the water. Plankton, for example, absorb violets and blues, the colors transmitted best by clear water. The amount of material suspended in some harbor water is frequently sufficient to alter the transmission curve completely; not only is very little light transmitted, but the long wavelengths may be transmitted better than the short, a complete reversal of the situation in clear water.

Color vision under water, whether for the visibility of colors, color appearances, or legibility, is thus much more complicated than in air. Accurate underwater color vision requires that divers know the colors involved, understand the sensitivity of the eye to different colors, know the depth and underwater viewing distance, and are familiar with the general nature of water and the characteristics of the specific waters involved. Information is available from several investigations about which colors can be seen best and which will be invisible under water.

Changes occur too in the appearance of colors under water. For example, red objects frequently appear black under water. This is readily understandable when one considers that red objects appear red on the surface because of reflected red light. Since clear water absorbs the red light preferentially, at depth no red light reaches the object to be reflected, and therefore the object appears unlighted or black. In the same way, a blue object in yellowish-green water near the coast could appear black. Substances that have more than one peak in their reflectance curve may appear quite different on land and in the sea. Blood is a good example; at the surface a reflectance maximum in the green is not noticeable because there is a much larger one in the red. At depth, the water may absorb the long wavelength light and blood may appear green. The ghostly appearance of divers in 20 to 30 feet (6.1 to 9.1 m) of clear water is another example of the loss of red light.

In general, less and less color is perceived as the depth and viewing distance under water are increased, and all objects tend to look as though they are the same color (the color that is best transmitted by that particular body of water). Objects must then be distinguished by their relative brightness or darkness. Many of the most visible colors are light, bright colors that give good brightness contrast with the dark water background. If the background were different (for example, if it were white sand), darker colors would have increased visibility. Fluorescent colors are conspicuous under water because fluorescent materials convert short wavelength light into long wavelength colors that are rarely present under water, which increases the color contrast.

The use of color coding under water is complicated by these changes in color appearance, and only a few colors can be employed without risk of confusion. Green and orange are good choices, since they are not confused in any type of water. Another practical question concerns the most legible color for viewing instruments under water; the answer depends on many conditions, which are specified in Human Engineering Guidelines for Underwater Applications. In clear ocean water, most colors are equally visible if they are equally bright, but in highly turbid harbor waters, red is best for direct viewing and green is best for peripheral or off-center viewing.

Insufficient Light
Attenuation and scatter dramatically reduce the amount of natural light available under water, restricting natural daylight vision to a few hundred feet under the best of conditions and to 1 to 2 feet (0.30 to 0.61 m) or less under the worst or highly turbid conditions. If there is not enough light (without an auxiliary dive light) for daylight vision, many visual capabilities that we take for granted in air will be greatly different; this includes good acuity, color vision, and good central or direct vision. In a low-light situation, acuity is very poor and the diver will be unable to read; he or she will have no clear vision, because all objects will appear white, gray, or black; the diver will have to look off center to see rather than looking directly at an object. Moreover, in order to see at all, the diver must darkadapt.

In air, an individual can gradually adapt to nighttime light levels during twilight and probably not notice the change in vision; however, a diver may go directly from bright sunlight on the boat into a dark underwater world and be completely blind. To function effectively, the diver's eyes must adjust to the dim illumination for as long as 30 minutes if he or she has been in bright light. Some adaptation will take place while the diver descends, but the rate of descent cannot be slow enough to make this a practical solution, and other techniques are required. This is especially important during dives in which the bottom time is short and visual observation important.

The most effective way to become dark-adapted is to remain in the dark for 15 to 30 minutes before the dive. If this is impossible, red goggles are recommended. The night vision system of the eye is relatively insensitive to red light; consequently, if a red filter is worn over the face plate before diving, the eyes will partially adapt and at the same time there will be enough light for the day vision system to continue to function. The red filter should be worn for 10 to 15 minutes and must be removed before the dive. Because high visual sensitivity is reached sooner when this procedure is used, visual underwater tasks can be performed at the beginning of the dive instead of 20 to 30 minutes later. If it is necessary to return to the surface even momentarily, the red filter should be put on again, because exposure to bright light quickly destroys the darkadapted state of the eye.

Colors under water
It has been experimentally established that seawater alters different colors in the same way as a blue lens. Ultraviolet rays reach farthest, whereas infrared ones are absorbed literally centimeters under the surface of the water. At a depth of 5m water lets through up to 45% of the blue sector of the spectrum, in the same time absorbing up to 60% of the re d sector. That is why the surface layer looks blue-green. 

Visibility decreases in murky water – tiny solid particles quickly scatter the light rays. In rivers or at harbors underwater visibility decreases so much that the diver practically sees nothing even in shallow waters. Moreover, while performing work under water, the diver himself makes the water murky.

In most cases, divers work in the dark, groping around, which requires good orientation skills and an ability to work while little or no light is available.

Certain types of underwater work are difficult to accomplish even if the divers are trained to work in the dark. In such cases, artificial illumination is recommended. Artificial light, however, discloses the work that is being performed and is dangerous if used in war time.