Furthermore, there will be a great deal more absorption from the other 

 elements present in water, notably oxygen and nitrogen. 



It can be seen, therefore, from this and the preceding section, 



that there is no hope of finding a window at any frequency higher than 



15 ° 



the ionization frequency of water (~3xlO cps = 1100 A), At frequencies 



below this, the photoelectric effect disappears. The rest of the report 



will be concerned with this lower frequency region. 



Obsorption Processes in the Visible Region 



A. Scattering of light by Na and Cl ions (Thomson scattering ) 



When light falls on free charged particles, it sets them into 



oscillation and causes them to emit radiation. The cross section for this 



1 . 

 process is 



+ - -33 2 -34 2 



For Na and Cl , cr is 3o3xl0 cm and 1.5x10 cm respectively. The 



mean free path of the radiation for this process X = ■— : (where n is the 



na 



19. 



ion concentration, here the NaCl concentration in the ocean, 3.6x10 ) is 



. 1 ~ ,„13 



X = Tc, lo^ = 10 cm. 



3.6x10 (1.5+3. 3)xl0 



Therefore, any absorption of the incident radiation due to this process is 

 negligible. If, as is probable, the Na ion forms a complex in water, the 

 effect will be even less. 



B. Scattering by Small Particles 



„ The scattering of light by small particles has been discussed by 



3 4 



Mie , Bom and Stratton . The mathematics involved is lengthy and laborious, 



and only an indication of what goes on will be sketched here. Some experi- 

 mental results will be presented below. 



The simplest model is that of a small sphere upon which is incident 

 a plane electromagnetic wave. To describe the scattering, solutions of 

 Maxwell's equations must be found that add up to give a plane wave at large 

 distances from the sphere and also are of the proper form to satisfy boundary 

 conditions at the surface of the sphere. It turns out that the scattering 

 cross section can be represented by a series of functions which depend on the 

 dielectric constant and conductivity of the sphere, and the ratio, a/X, of 

 sphere radius to wavelength. The first term of the series is seen to be the 

 radiation emitted by an electric dipole, while the other terms correspond to 

 emission by higher multipoles. 



-4- 



