14 BIOLOGICAL EFFECTS OF RADIATION 



cm.-S the second in centimeters per second or cm./sec, the frequency, 

 therefore, in reciprocal seconds (symbol sec "O. The reader will seldom 

 see numerical values of frequency cited. 



d. The c.g.s. unit of energy is the "erg." The energy of a photon 

 of hght of frequency v is computed in ergs by multiplying v expressed in 

 sec.-i by the value of h expressed in c.g.s. units (gm. cm.^ sec.-^), which is 

 6.55 • 10-27. The erg is far too large a unit for convenience in dealing 

 with any portion of the spectrum, and the customanj unit of energy 

 is the "electron volt" (symbol ev) equal to 1.59 • IQ-^^ gj-g Sometimes 

 this is called the "equivalent volt," and often simply the "volt of energy" 

 or "volt," a usage much to be reprehended. As the name imphes, it is 

 the amount of kinetic energy acquired by an electron in passing unim- 

 peded between two points of which the latter is at a potential one volt 

 higher than the former. In the gamma-ray region, photon energies are 

 sometimes expressed in milUons of electron volts (symbol mev). 



To get photon energy in electron volts from wave-length in Angstroms, 

 it is often convenient to remember that photons of one electron volt 

 correspond to waves of 12,337 A. If the reader finds it easier to hold in 

 mind the number 12,345 than the other, he commits no serious error in 

 doing so. 



THE REGIONS OF THE SPECTRUM 



While the several regions of the spectrum derive their names from 

 physiological and historical accidents rather than from fundamental 

 reasons, it is important to have some notion of their locations and 



extents. ^ 



The visible spectrum extends from about 3600 A at the end of the violet 

 to about 8500 A at the end of the red, the exact limits varying from one 

 eye to another. The corresponding photon energies are about 3.4 and 

 1.4 EV, respectively. 



From the red end of the visible spectrum, in the direction of increas- 

 ing wave-lengths, extends the infra-red, beyond which lies the spectrum 

 of Hertzian waves generated by oscillating electrical circuits. Until 

 about 1924 there was a gap between these ranges — i.e., an intermediate 

 range of wave-lengths corresponding to no radiations which had ever 

 been observed — and the infra-red was defined as extending to the short- 

 wave end of this gap. Subsequently the gap was closed, and this con- 

 venient definition lost; the infra-red is now commonly considered as 

 extending to about 1000 n or one millimeter, corresponding to a photon 

 energy of only about 0.001 ev. People sometimes speak of the "near" 

 or the "far" infra-red, but the distinction is extremely loose, unless 

 perchance the near infra-red be defined as that range over which photo- 

 graphic plates are sensitive. This range is constantly being extended 



