Introduction 



The main purpose of this report is to examine the theoretical and 

 experimental evidence which exists on the absorption of electromagnetic 

 radiation by water in the hope of finding windows through which the radi- 

 ation may propagate with little attenuation. 



There were two principal reasons for reopening this subject: 



1) In the last two or three years, there have been great advances 

 made in the detection of radiation, especially in the infrared 

 region of the spectrum; 



2) There does not seem to be any adequate investigation of water 

 absorption in the infrared using modern techniques. 



There have been measurements covering various ranges of frequency, and 

 these will be reviewed below„ Furthermore, the reasons for the known 

 great absorption in the infrared do not appear to be adequately explained 

 on a theoretical basis. 



The term absorption will also include any scattering process by which 

 a beam of radiation is attenuated. There are a number of mechanisms of 

 absorption, and each will be considered in the next sections. Following 

 this, a review of the experimental work on absorption in water will be pre- 

 sented. A summary of absorption throughout the whole spectrum will be 

 given in the next section; and in the final section of this report, a 

 discussion and recommendations for any future work will be made. 



The Absorption Processes at Very High Frequencies 



The processes will be considered roughly as a function of the frequency 

 at which a specific process is most effective, starting with the highest 

 frequencies. 



1 R 

 A. Pair Production and Compton Effect (> 10 cycles/sec) 



These processes are effective only at extremely high frequencies, 



1 Q 



in the region of the spectrum usually referred to as gamma rays (10 cps 

 and up). Absorption by pair production arises when a gamma ray disappears 

 and gives rise to the creation of a positron and an electron. This process 

 must take place near a nucleus in order to conserve momentum. The Compton 

 effect arises from the scattering by electrons of the electromagnetic 

 radiation. 



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