PRINCIPLES OF RADIOLOGICAL PHYSICS 17 



product of the velocity of propagation of the radiation and of the duration 

 of one oscillation of the current. The velocity of propagation c is given 

 very approximately by 



c = 3 X 10' meters/sec = 3 X 10 1" cm/sec (7) 



in empty space, in air, and in other gases. The duration of one oscillation 

 is the reciprocal of the frequency, v, of oscillations per second. Conse- 

 quently, the basic relation between wave length and frequency is 



X = c/v (8) 



The frequency of electromagnetic radiation can have any value in 

 principle, according to present knowledge. Radiations of frequencies 

 up to more than 10"^^ cycles/sec, whose wave lengths range down to about 

 10"^* cm, have been studied experimentally. 



Radiation of "low" frequency succeeds in driving electric currents back 

 and forth through macroscopic portions of matter such as wires or other 

 conductors. If the frequency is higher than, say, 10^- cycles/sec, the 

 currents cannot travel over macroscopic distances within a cycle of oscil- 

 lation of the disturbance, but currents confined within atoms become 

 important. Low mass and high rigidity make atomic and subatomic 

 structures particularly apt to oscillate with very high frequencies (above 

 10^^ cycles/sec). Atomic charges perform forced oscillations under the 

 driving influence of a monochromatic radiation. The intensity of these 

 oscillations depends on the same circumstances which govern the forced 

 oscillations of any mechanical system. 



Any mechanical system which is stable but susceptible to elastic 

 deformation is capable of performing free elastic oscillations with charac- 

 teristic frequencies of its own. The electrically charged particles within 

 matter, primarily the atomic electrons, constitute an elastic system whose 

 characteristic frequencies are very high. Forced oscillations become 

 extremely intense when the frequency of the driving force coincides with 

 a characteristic frequency of the oscillating system. This particularly 

 strong reaction of atomic charges to a specific radiation is called "reso- 

 nance." Similarly a very intense current is induced in the wire of an 

 oscillating circuit by the action of lower freciuency radiation when the 

 circuit is "tuned" to oscillate with the radiation frequency. 



The induction of intense resonating currents within the atoms of a 

 material or within the wires of a macroscopic circuit requires the expendi- 

 ture of energy by radiation. Therefore the occurrence of resonance is 

 associated with the absorption of radiation. 



Radiation of any particular frequency propagates easily only through 

 those materials whose internal currents do not respond easily to radiation 

 of that frequency. Even in the absence of resonance, the proximity of 

 the radiation frequency to the characteristic frequencies of internal cur- 



