We see because light enters the eye triggering nerve impulses in the brain. We interpret these impulses because we are accustomed to the way light acts in air. In water light acts differently. For a diver to see clearly he must have an air space in front of his eyes, this air space is provided by the mask. When light rays pass from one medium, the water, to another medium, the air in the mask, the rays are bent. Straight lines sometimes appear curved and objects appear larger and closer than they really are. This is called REFRACTION. Light rays are also moved or REFLECTED from an object in the water, to your eye in the air. This bending of light sends a false impression which is received by the brain. This false impression is as to position and size of the object. This distortion is usually taken to be 25% to 33% larger and 25% to 33% closer than reality depending on the density of the water and thickness of the glass..
Diffusion is the result of light being deflected by the water and foreign particles suspended in the water. This reduces the illumination but it can also be beneficial. Diffusion is responsible for throwing light in otherwise dark corners. If not for this effect, any object not direct line with the sun, would have practically no illumination.
Turbidity is caused by too much suspended particles in the water. In surf areas it may be sand, caused by the breakers, in lakes it can be caused by wind, boaters, divers, or anything that stirs up the bottom, also it could be pollutants in the water. Natural turbidity occurs in freshwater at different times of the year when the body of water ''turns''. It is a process where the colder bottom water rises to the top of a lake. Relatively clear layers are sometimes found in turbid water near the bottom.
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