258 



BIOLOGICAL EFFECTS OF RADIATION 



1,650 



which absorbs all the incident radiation and reflects none. It gives, 

 accordingly, the maximum radiation when heated. A heated enclosure 

 such as a furnace with a small opening furnishes a practical means of 

 obtaining this black-body radiation. Further details concerning black- 

 body radiation are given on page 135. 

 It is evident from Fig. 4 that a heated 

 solid, a tungsten filament, for exam- 

 ple, offers a good source of light in the 

 visible region of the spectrum but not 

 in the ultra-violet. 



When atoms or molecules in the 

 gas phase are subjected to extra en- 

 ergy, the radiation may be emitted at 

 specific wave-lengths, as distinguished 

 from the continuous radiation of 

 heated solids. The passage of an 

 electrical current between metal elec- 

 trodes causes the emission of a dis- 

 continuous spectrum as in the case of 

 the well-known mercury arc or iron 

 arc. 



TYPES OF SPECTRA 



Spectra are classed as 



2 5 



Wave- 



emission 



spectra and absorption spectra. 

 Absorption spectra are obtained when 

 white light of continuously varying 

 wave-lengths is passed through ab- 

 sorbing material. Light of certain 

 wave-lengths passes through un- 

 affected, while light of other wave- 

 lengths is able to displace electrons (or 

 atoms) in the molecules of the absorb- 

 ing material. When the light rays 



Fig. 



4- 5 



ength (m^) 



4. — Distribution of radiant energy are spread out in a spectrum with a 



emitted by a heated solid. 



prism or grating, those wave-lengths 

 which have been absorbed are missing and a dark region results. The 

 darkest regions in the absorption spectrum correspond to the greatest 

 absorption and the most probable displacements within the molecule. 

 As a rule, the absorption spectra, particularly in the case of solutions, 

 are not so definite and the lines are not so sharp as in the case of emission 

 spectra. 



Band Spectra. — In the case of simple atoms, electron displacement 

 usually leads to fine lines and discontinuous spectra. When molecules 



