12 BIOLOGICAL EFFECTS OF RADIATION 



If all of these assumptions are valid, it follows that E^^, must conform 

 to the equation, 



and consequently (a) Vo must he a linear function of\-\ its slope the same 

 for all metals; (6) Fo must be independent of the intensity of the light (which 

 does not appear anywhere in the equation) ; and (c) the total number of 

 electrons emerging per unit time from the metal (the quotient of im by e) 

 must be proportional to the number of photons falling per unit time on 

 the metal and therefore to the intensity of the light. 



All of these expectations are fulfilled. It was the second of these 

 laws— the law that the intensity of the light is without influence on the 

 energy of the escaping electrons — which served as the earliest incon- 

 trovertible evidence that the behavior of hght cannot wholly be explained 

 by conceiving it as a pure wave motion. For in trying to interpret the 

 photoelectric effect in the classical manner, one must imagine the electro- 

 magnetic waves entering into the metal and setting the indwelling elec- 

 trons into forced oscillations, by virtue of the continuously varying 

 field strength and energy which accompany them. The oscillations of 

 such an electron would grow steadily wider; the speed with which it 

 dashed through its middle position would grow larger and larger; at last 

 it would be torn from its moorings. One would predict that the greater 

 the intensity of the light, the greater the energy acquired by the electron 

 in each cycle of its forced oscillation would be, the greater the energy 

 with which it would finally break away. But E^,, (as indeed the whole 

 distribution in energy of the photoelectrons) is independent of the 

 intensity; it is as though the waves beating upon a beach were doubled 

 in their height and the powerful new waves disturbed four tunes as many 

 pebbles as before, but did not displace a single one of them any farther 

 nor agitate it any more violently than the former gentler waves did the 

 pebbles that they washed about. 



The second and the third of the foregoing laws thus speak powerfully 

 for the doctrine that, at least in respect of absorption, a beam of Ught is 

 to be regarded as a hail of corpuscles; and they justify us in using the 

 linear relation between Fo and \-^ which now I write in the form 



Fo = 4-' + ^4^ (^^^ 



A 



as a basis for determining the constant h; for h is equated simply to 

 eA i/300c. Incidentally, the theory as a whole requires that {eWa - eWi) 

 should vary from one metal to another just as does the Volta potential 

 or contact potential, and is further strengthened by the fact that A 2 



