16 



RADIATION BIOLOGY 



the current; in fact, it is proportional to the fourth power of this rate of 

 change. 



A radiative disturbance exerts the same kind of action upon matter as 

 electromagnetic induction. It exerts an electric force upon all electric 

 charges within matter and a magnetic force which tends to push all cur- 

 rents sideways. 



However, the effects produced by these actions vary greatly, depending 

 upon the rapidity with which the electric and magnetic forces vary in 

 the course of time. Not only the methods of detection of electromagnetic 

 radiation but also the classification of the various types of this radiation 

 depend on the analysis of the time variations of radiative disturbances. 



Therefore this analysis deserves first 

 consideration. 



1-3 a. Frequency and Wave Length. 

 Electromagnetic disturbances fre- 

 quently follow an oscillatory, ahnost 

 perfectly sinusoidal, course. This ob- 

 tains, for example, for light of a defi- 

 nite color. For this reason sinusoidal 

 radiative disturbances are called 

 ' ' monochromatic. ' ' 



Monochromatic radiation arises 

 from a current that performs sinusoi- 

 dal oscillations. In turn, monochro- 

 matic radiation induces sinusoidally 

 oscillating currents within matter. The rate of variation of a sinusoidal 

 disturbance is characterized by the number of its cycles of oscillations per 

 unit time, i.e., by its frequency. 



The study of monochromatic radiations of different frequencies serves 

 as a basis for the study of any electromagnetic radiation because any 

 radiation may be regarded as the combination of monochromatic com- 

 ponents. In fact, no radiation ever follows a perfectly sinusoidal course; 

 actual "monochromatic" radiations are, at best, aggregates of ideally 

 monochromatic components whose frequencies are very nearly equal. 



At any instant during the propagation of a disturbance away from an 

 oscillating current, the effect of successive pulsations of the current have 

 reached points at various distances from the source. Figure 1-12 repre- 

 sents diagrammatically the strength and direction of the force that acts on 

 electric charges placed at various positions around a vertical "antenna." 

 The diagram displays the wavelike character of the propagation of 

 radiation. 



The distance between successive layers of points where the electric 

 force has, for instance, maximum strength and equal direction is called 

 the "wave length of the radiation." The wave length, X, equals the 



Fig. 1-12. Schematic diagram of the 

 distribution of the electric field about 

 an antenna. 



