The Sea-water and its Physical and Chemical Properties 



53 



of these particles is very small compared with the wavelength of light and if the con- 

 centration is not too large, the scattering is due to pure diffraction following Rayleigh's 

 law; according to this the reduction in intensity of the incident light is inversely pro- 

 portional to the fourth power of the wavelength. Amongst the phenomena due to 

 scattering is included that known as the Tyndall ejfect, where a beam of light passing 

 through a turbid medium produces a more or less intensive illumination of those por- 

 tions in the medium affected by light. This is due to reflection and scattering of the 

 light by the suspended particles. Since the shorter wavelengths are more strongly 

 scattered, the Tyndall-light is bluish. The water molecules themselves can be regarded 

 as scattering particles. Thereby one thought to explain also the blue colour of the 

 scattered light in pure water. However, it has later been recognized that a direct scatter- 

 ing by the water molecules can hardly occur since there are too many compressed 

 into a small space and the distances between them are too small relative to their 

 diameter. According to the theory of Smoluchowski irregular molecular movements 

 give rise to an optical inhomogeneity (streaks; Schlieren) of very small dimensions 

 and are therefore responsible for the scattering of light. 



Table 19. The energy distribution in the spectrum of sunlight after passing 

 through water layers of different thickness 



The only natural parallel radiation occurring in the upper surface of the sea is 

 direct sunlight. On passing through water the spectrum of sunlight undergoes great 

 changes. Schmidt (1908), on the basis of the extinction values of Aschkinas and values 

 according to Langley for the distribution of radiation energy from the sun on the 

 surface of the sea, has calculated the spectrum of the sunlight at different depths and 

 obtained the values given in Table 19 for water layers of difiTerent thickness; the total 

 radiation from the sun incident on the surface of the sea is taken as 1000 (Fig. 28). 

 The total extinction for different layer-thickness is given in Table 20. The reduction 

 in intensity of sunlight after passing through very thin layers of water is quite consider- 

 able. For a layer 1 cm thick, wavelengths >l-5^t are completely eliminated and the 

 spectrum extends only to 0-9 /x. For layers 100 m thick the remaining energy has 

 fallen to less than 1-5%. 



