Co Molecular Scatterin g 



From the microscopic point of view, an electromagnetic wave 

 incident on a substance distorts the molecules of the substance, thereby 

 inducing molecular di pole moment s. These induced dipoles can be considered 

 as vibrating in a vacuumo They radiate 1) a wave which exactly cancels 

 the incident wave in the substance, 2) a reflected wave, 3) a refracted 

 wave, and 4) a scattered wave. The details of this process are presented 

 in the book by Borno It is shown there that the first three processes 

 depend only on the average induced dipole moment per unit volume, while 

 the fourth process depends on deviations from this average moment such as 

 would be produced by density f luctuationSo 



The intensity of the scattered radiation J is 



T (277") „2, .2 

 J - 4iT — J— p (AN) 



X 



. (277) 12 



K 



where p is the induced dipole moment, AN is the fluctuation in the number 



of molecules per unit volume from the average value, p the density, K the 



compressibility. It depends on the inverse fourth power of the wavelength 



as in the case of scattering by particles. However, this scattering occurs 



in the purest substances. 



2 

 The cross section is obtained by dividing J by CE /4i7; 



25677 1 I rp l^ 



cX 



since p = a E, where a is the polarizability of the molecule and E the inci- 

 dent field. For radiation of wavelength 6000 A incident on water at 20°C, 



CT ~ 7xlO"^^K = 3. 5x10"^ » 



1 15 

 The radiation mean free path is — = 10 cm, so that this effect is negli- 



gible. 



At the triple point of water, T = .0075°C and p - 4.58 mm, K 

 becomes extremely large and water becomes practically opaque. 



Absorption and Dispersion for Ultra-Violet and Infrared Frequencies 



Up to this point of the report, scattering processes have been con- 

 sidered (except for the high energy processes of photoelectric absorption 

 and pair production). This section will be concerned with the absorption 

 and dispersion(the variation of the index of refraction with frequency) of 

 radiation. Very general relations exist between the refractive index and 

 the absorption coefficient that enable one to determine the absorption if 



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