192 



Light 



parts of the spectrum. Measurements of illumination made with a 

 photometer placed in a watertight case and lowered into our imag- 

 inary lake would reveal the changes presented in Fig. 6.5 At a depth 

 of 70 m the blue component of sunlight has been reduced to about 

 70 per cent of its intensity at the surface. At the same depth the 

 yellow component of sunlight has been reduced to 6 per cent of its 

 incident value. The orange and red components have been ex- 

 tinguished very much more rapidly. At a depth of 4 meters, red 

 light has already been diminished to about 1 per cent of its surface 

 intensity. 



Percentage of incident light 

 5 10 



50 



100 

 m 



10 



20 



30 2 



40 



50 



60 



70 



Fig. 6.5. Reduction in intensity ( logarithmic scale ) of the color components of 



sunlight (indicated by initial letters) at increasing depths (hncar scale) in a lake 



of optically pure water. (Clarke, 1939, AAAS Pub. No. 10.) 



Since we know tliat the daylight incident upon the surface of our 

 lake differs markedly in intensity in different parts of the spectrum, 

 we are dealing with the unequal extinction of an unequal spectrum. 

 As shown in Fig. 6.6, the extinction by pure water of light at the two 

 ends of the spectrum is much more rapid than in the middle of the 

 spectrum. The blue component of light is also the most penetrating 

 in the clearest ocean and lake waters. At depths of 100 m or more 



