RADIATION OF ENERGY. Kl 



Again, the velocity of the electrons emitted by a highly polished metal plate 

 under the action of ultraviolet light is found to depend upon the frequency and 

 not upon the intensity of the exciting light. And on the other hand, the number 

 of the electrons emitted depends upon the intensity of the exciting light. These 

 results can be explained if we again assume that the energy of each electron comes 

 from a single impinging quantum. For then, with monochromatic light of a 

 given frequency, each quantum would impart to each electron a definite velocity 

 which would depend only upon the given frequency. 



Again, since the energy of a light quantum is directly proportional to the 

 frequency of the light, we see why, according to Einstein's theory, light of th« 

 lower frequencies would not produce the photoelectric effect, the energy of a 

 single quantum not being great enough. Lorentz 1 gives the following illustrat ion. 

 If we take for ultraviolet light 



v = 1.03 X 10 16 



we will have for the energy of a quantum of ultraviolet light 



e = hv = 6.7 X 10- 12 erg. 



According to Lenard, the energy of the emitted electron is equal to 



3 X 10- 13 erg. 



An ultraviolet quantum then has enough energy to produce the observed velocity 

 of the electron, and also enough to do whatever work is necessary to set it fro. 

 Einstein's hypothesis can also be used to explain the peculiar results obtained 

 by Stark 2 for the Doppler effect in canal rays. Since the particles in a beam 

 of canal rays have many velocities, varying from to a certain maximum valu< 

 it is to be expected that if the light emitted by the moving particles is observed 

 in a spectroscope, in the direction of their motion, a characteristic line of the gas 

 will be spread out, in a band, towards the violet end of the spectrum, the band 

 beginning at that point in the spectrum where the characteristic line normalh 

 exists. Stark, however, found that the band did not begin at this point but at 

 some distance to the right of it, and that it had two maxima of brightness. The 

 appearance of the band and the positions of the maxima can be explained if we 

 assume that the particles radiate light energy only in multiples of a quantum 

 No particle would then radiate energy unless upon a collision with a molecule its 

 kinetic energy was great enough so that it could emit one or more quanta. The 

 first maximum would then correspond to a velocity which would make the kinetic 

 energy great enough to produce an emission of one quantum, and the second maxi- 

 mum to a velocity which would make the kinetic energy great enough to produce 

 an emission of two quanta. Stark found that the ratio of the distances of the 



1 Loc. eU., p. 1249. 



1 Physiktdinche Zeitschrift, Jahrg. 9, p. 767, 1908. 





