As an example, the back scattering cross-section of a sphere of 

 dielectric constant e and infinite conductivity for a/X. « 1 is given 

 by only the dipole terra 



ira 



xlO 



It is seen that this is the famous Rayleigh scattering which accounts 

 for the blue color of the sky. 



Fig. 1 shows a plot of this cross-section vs a/\. 



/ a 

 The 1 7- j or Rayleigh scattering is shown by the dotted line„ It coincides 



with the solid curve for a/\ « 1. At about a/X. - „05, higher order multi- 

 pole terms become larger and fluctuate in phase and magnitude causing the 

 oscillations^ As a/\ becomes larger, a approaches the geometrical cross-section. 



As a sample calculation, suppose water containing one part per million 



by volume of infinitely conducting spheres scatter radiation of wavelength 



° o _4 



6000 A (visible region). For radii of 12000 A (^ 10 cm) the scattering 



2 

 cross-section is about geometric, wa „ The mean free path for this radius is 



/ 2 

 given by 1/n va where n is the number of scattering particles per cubic 



5 

 centimeter (2,5x10 in this case). Its value is 130 cm. Experimental results 



pertaining to the clarity of ocean water will be discussed below. 



