NATURAL LIGHT FIELDS 733 



of the overhanging foliage. They Hve in the "green shade." Figure 22.53, 

 taken from Seybold (1936), shows the spectral compositions of the light 

 field in the midst of a forest, compared with that at the edge. Character- 

 istic is the minimum at 650 m/x, clearly corresponding to the absorption 

 maximum of chlorophyll. A large part of radiations reaching the floor of 

 a forest are either infrared or deep red, scarcely visible to the eye and use- 

 less for photosynthesis. The total intensity of the light field under the 

 trees (400-700 m^u) is less than 10% of that of full sunlight above the forest 

 and can drop to as low as 1% in a dense pine forest (Seybold). 



An even stronger alteration in the intensity and spectral composition 

 of the light field occurs when sun rays pass through thick layers of water. 



Table 22. IX gives some data on the decrease in total intensity with depth. 

 The main cause for this drop in light intensity is the absorption by water 



Table 22.IX 

 Decrease in Light Intensity with Depth 



50 100 200 ft. 



Atlantic Ocean 



Brightness" 1076 114 37 4.4 



1 2 5 10 20 m. 



Titisee Lake 



Intensity-* 100 57 32 21 13 10 



Bodensee Lake 



Intensity*. 100 54 30 15 9 6 



« Beebe and HoUister (1930), Hulburt (1932). 

 '•Seybold (1936). 



itself. Some higher bands (overtones) of the vibrational spectrum of 

 water lie in the visible spectrum (the fundamental frequencies are in the 

 near infrared). They decrease in intensity from red to blue; in the violet 

 and near ultraviolet, however, the absorption increases again, probably 

 due to weak electronic bands. Figure 22.54 shows the extinction curve 

 of water in the region 360-800 m^u, according to the measurements of 

 Aschkinass (1895) and Sawyer (1931) (c/. Dorsey 1940). A water layer 

 10 m. thick reduces the light intensity at 640 m/x by the factor of 10; a 

 layer 1 m. thick does the same at 760 mju ; 100 m. are required for a similar 

 reduction at 440 m^. Because of this absorption in red, yellow and violet, 

 thick layers of pure water are bluish green in color. 



The absorption by natural waters in the blue and in the violet is usually 

 much larger than that by pure water, due partly to the presence of certain 

 inorganic ions (e. g., iron) and partly to that of organic matter (e. g., the 

 chlorophyll of the phytoplankton). This increased absorption at the 

 short-wave end of the spectrum gives natural waters a pure green or even 



