18 RADIATION BIOLOGY 



rents determines the speed of propagation of the radiation. Thus the 

 mode of interaction of radiation with any material appears to depend to 

 a great extent on the radiation frequency, and it seems natural to classify 

 electromagnetic radiation according to its frequency or wave length. 



Newton first utilized the different refraction of light of different fre- 

 quencies to resolve light into its multicolored components. He called 

 the resulting pattern of colors a "spectrum." The analysis of radiation 

 into its monochromatic components is now generally called "spectral 

 analysis." 



l-3b. The Potency of Electromagnetic Radiation. The effects of reso- 

 nance establish a connection between the frequency of radiation and its 

 ability to affect various portions of matter, because the energy of radia- 

 tion is absorbed selectively when the internal currents happen to resonate 

 with the radiation. 



However, a much more striking connection between the frequency 

 and the potency of radiation appears when the effect of radiation on 

 individual atomic particles is observed. Such observations can be made, 

 for example, in the study of the "photoelectric effect," in which phenome- 

 non electrons escape from matter under the influence of monochromatic 

 light. Each ejected electron turns out to have received from the light a 

 definite amount of energy which is proportional to the light frequency. 

 The proportionality constant (/i), which is called the "Planck constant," 

 always has the same value: 



h = 6.62 X 10-" erg sec = 4.14 X IQ-^'^ ev sec (9) 



For example, ultraviolet light with a wave length of 2500 angstrom units 

 (A), equal to 2.5 X 10~^ cm, has a frequency v of 



3 X 1010/2.5 X 10-^ = 1.2 X 10'^ cycles/sec 



according to Eqs. (7) and (8). The energy which this light imparts to an 

 electron equals the product 



hv = 4.14 X 10-1^ X 1.2 X 101-^ = 4.96 ev (10) 



A certain minimum amount of energy is required to eject any electron 

 from the atoms of matter. Therefore low-frequency radiation, which can 

 impart only a small amount of energy to an electron, may be wholly 

 incapable of ejecting electrons. For example, light of 2500 A which 

 imparts to electrons 4.96 ev at a time, can produce no photoelectric effect 

 in any monoatomic vapor except cesium (for which 3.87 ev is sufficient). 

 However, this light is capal)le of extracting electrons from a number of 

 solid metals because electrons are less strongly bound within solid metals 

 than within vapor atoms of the same elements. The lowest frequency of 

 radiation capable of ejecting electrons from a material is called the 

 "photoelectric threshold" of that material. 



