22 



RADIATION BIOLOGY 



radiation components of different frequencies at various temperatures. 

 (The characteristic frequencies of oscillation of the individual atoms in 

 the material do not appear in this spectrum because the atoms are not 

 isolated but are continuously and variously disturbed by other adjacent 

 atoms under the influence of thermal agitation.) 



Radiation of frequency falling within narrow limits can be selected 

 from a beam containing a wide variety of frequencies by means of 

 "monochromators" such as a spectroscope. A cruder chromatic analysis 

 can be performed with filters which absorb light of different frequencies 



24 

 22 

 20 



18 



>- 16 



to 



z 14 

 tu 



h- 



? 12 



10 



< 



10,000 20,000 30,000 40,000 50,000 



WAVE LENGTH, A 



Fig. 1-13. Spectral distribution of the radiation from ovens at different temperatures. 



to a different extent. These methods are naturally wasteful of energy 

 since they utilize only a small fraction of the radiation available. 



For better efficiency advantage is taken of the fact that excited atoms 

 or molecules in rarefied gases emit light of characteristic frequencies. 

 The atoms are excited by the impact of electrons, usually in an electrical 

 discharge. With suitable technical arrangements it is possible to convert 

 into radiation of desired wave lengths a substantial fraction of the energy 

 imparted to the discharge (Fig. 1-14). This method is now being used 

 increasingly also for illumination purposes. 



Certain substances have the property of absorbing light of compara- 

 tively high frequencies and of reemitting it as light within a limited range 

 of lower frequencies. This phenomenon is called "fluorescence." In the 

 fluorescent lamps a gas emits primary radiation which is ultraviolet and 

 is turned into visible by a fluorescent material which is part of the lamp 

 wall. 



