34. How deep in the ocean can one see with natural sunlight? 



Even when perfectly clear, water is at least a thousand times more 

 opaque than air, because of the density difference. The depth to which 

 one can see in the ocean is dependent on the amount of suspended mat- 

 ter and the angle of sunlight. Underwater visibility is best at noon when 

 the sun is directly overhead; at that time about 98 percent of the light 

 penetrates the sea surface. When the sun is at an angle of 10 degrees 

 above the horizon, only 65 percent of the light penetrates; the rest is 

 reflected. 



Tropical waters usually have high transparency; the Mediterranean 

 Sea, particularly the eastern section, is also noted for its good trans- 

 parency characteristics. 



As a diver descends into the ocean, the first change he notices is that 

 everything appears to be blue-green; when he approaches the 100-foot 

 level, it becomes impossible to distinguish colors. Light appears to come 

 from all directions and there are no shadows. Cousteau reports that at 

 300 meters the pale blue lighting is hardly sufficient to define the shapes 

 of objects a short distance away. 



Sometimes horizontal visibility is better at greater depths because of 

 the higher amounts of suspended materials in surface waters. Italian 

 divers working on the liner EGYPT southwest of Brest, France, reported 

 that visibility diminished as they went to a depth of 66 feet, then im- 

 proved. Light faded as they reached the wreck at 396 feet; at that depth, 

 visibility was 6 feet. 



Caidin, Martin 



Hydrospace, E. P. Dutton and Company, 1964. 

 Cousteau, Capt. J. Y. 



The Silent World, Harper and Brothers, 1953. 

 Rebikoff, Dimitri 



Underwater Photography, American Photographic Book Publishing 



Company, 1965. 



36 



