soMi- co.wir.Mi'Oh'.iRv .u>r.ixci:.s ix I'livsics rii .11.1 



however to Ik? discovered Liter, as I sliall presently mention Imi llu' 

 presence of radiation of a new sort, come into beinp hy virtue of tlie 

 encounters between tlie original radiation anrl free electrons. We 

 have not encoinitered an>thinK of this sort heretofore. When a 

 qiiantiuii of radiant energy releases an electron from an atom, it dies 

 completely and confers its entire energy upon the electron. The 

 disposal of its momentum gives no trouble, for as I have mentioned 

 the atont takes care of that. When the electron is initialK' free. 

 and there is no atom to swallow up the momentum of the radiation, 

 it c.innot be ignored in this simple fashion. For if the quantum did 

 utterly disappear in an encounter with a free electron, the velocity 

 which the electron acquired would have to be such that its kinetic 

 energy and its momentum were separately equal to the energy and 

 momentum of the ciuantum; but these distinct two conditions would 

 generally be impossible for the electron to fulfil. Hence in general, a 

 quantimi possessed of momentum cannot disappear l)y the process of 

 transferring its energy to a free electron, whatever may be the case 

 with an electron bound to a massive atom. This reflection might 

 easily ha\e led to the conclusion that radiation .md free electrons can 

 have nothing to do one with the other. 



What actually happens is this: the energy and the momentum 

 of the quantum are partly conferred upon the electron, the residues 

 of each go to form a new quantum, of lesser energy' and of lesser 

 and differently-directed momentum, hence lower in frequency and 

 deflected obliquely from the direction in which the original quantum 

 was tiioving. The encounter occurs much like an impact between 

 two elastic balls; what prevents the analogy from being perfect is, 

 that when a moving elastic ball strikes a stationary one, it loses 

 some of its speed but remains the same ball, whereas the quantum 

 retains its speed but changes over into a new and smaller size. It 

 is as though a billiard-ball lost some of its weight when it touched 

 another but rolled off sidewise with its original speed. I do not 

 know what this innovation would do to the technique of billiards, 

 but it would at all events not make technique impossible; the result 

 of an impact would still be calculable, though the calculations would 

 lead to a new result. The rules of this microcosmic billiard-game 

 in which the struck balls are electrons and the striking balls are 

 (|uanta of radiant energy are definite enough to control the conse- 

 (juences. The rules are these: 



Conservation of energy requires that the energy of the impinging 

 quantum, hv, be equal to the sum of the energy of the resulting 

 quantum, hv , and the kinetic energy K of the recoiling electron. For 



