124 BIOLOGICAL EFFECTS OF RADIATION 



the velocity is reduced, the wave-length modified, but the frequency 

 remains unchanged. Furthermore, in absorption and emission — 

 phenomena of interaction between radiation and matter — the relations 

 are most readily expressed in terms of frequency. These observations 

 constitute a strong argument in favor of expressing our data in terms of 

 frequency. Nevertheless, because of established usage, the preponderant 

 amount of existing data is to be found in terms of wave-length, the wave- 

 lengths being evaluated for either free space or air. For our purpose the 

 difference is not significant. While, for convenience, we shall follow the 

 established usage of speaking in terms of wave-length, immediate conver- 

 sion to frequency may always be made and will sometimes be desirable. 



ELECTROMAGNETIC SPECTRUM 



While most of our conceptions are based upon those observations of 

 which we are cognizant through our physiological senses, experimenta- 

 tion in recent years has shown that this visible range of wave-lengths 

 constitutes an extremely small portion of the great electromagnetic 

 spectrum for which we have data through physical measurements. 

 Different ranges of wave-lengths give rise to such markedly different 

 observable effects that they have been given names associated with the 

 effects produced. Only recently have the fundamental characteristics 

 common to all radiation been recognized and enabled us to arrange a 

 fairly complete assignment according to wave-length, extending from 

 0.00002 or 0.00003 A to 20,000 meters, a spread of 19 orders of magnitude. 

 Thus, on a common logarithmic scale, the spectrum covers a range of 

 almost 19 units. This is shown, together with a number of related 

 physical quantities, in Fig. 1 (27). Of this tremendous range, we shall 

 restrict ourselves in this section to a single order of magnitude, namely, 

 from 0.2 to 2.0 y. (2000 to 20,000 A), or one unit on the logarithmic 

 scale. 



In biological material, water plays such a dominant role that its 

 characteristic absorption of radiation is a matter of great importance. 

 Its rapid increase of absorption to wave-lengths shorter than 0.2 ^i and 

 to wave-lengths longer than 2.0 /i sets a natural limit to the range of wave- 

 lengths to be considered. Thus, the range of wave-lengths under con- 

 sideration represents an interval of relative transparency for water. 

 The outstanding characteristic of matter with respect to radiation in this 

 region is that of high selectivity, that is, rapid changes of absorption with 

 varying wave-length. It is from this selectivity that the great variety 

 of effects, both biological and physical, arise. At the same time, it is 

 this very selectivity which makes the problems of measurement difficult 

 and which has led to a considerable degree of confusion. 



