196 



Light 



Another way in which Hght is changed by water is in its angular 

 distribution. The direct beam of hght from the sun is bent by re- 

 fraction toward the perpendicular at the water surface. Since waves 

 or ripples almost always exist in natural situations, the light from the 

 sun tends to be broken up to a considerable extent in passing through 

 the surface. Within the water itself further diffusion is produced by 

 scattering. When larger amounts of suspended materials are present, 

 scattering proceeds at a very rapid rate, with the result that measur- 



Percentage of surtace light 

 1.0 5 10 



50 100% 



Fig. 6.8. Comparison of the transparencies of representative natural waters. 

 Curves based on the average extinction rate for the yellow-green (5000-6000A) 

 component of dayhght. (1) Midge Lake, Wis., (2) Trout Lake, Wis., (3) Gun- 

 flint Lake, Minn., (4) Woods Hole Harbor, (5) Thatcher Pass, San Juan Islands, 

 and Buzzards Bay, Mass., (6) Vineyard Sound, (7) Baltic Sea, (8) Crystal Lake, 

 Wis., (9) English Channel, (10) Gulf of Maine, (11) off Vancouver Island, and 

 off Nantucket Shoals, (12) Crater Lake, Ore., (13) Gulf Stream, (14) Caribbean 

 Sea, (15) distilled water. (Clarke, 1939, AAAS Ptihl. No. 10.) 



