i035Fig. 35—Showing how photo-electric stopping potentials of different metals are compared by rotatingandin vacuointo the position of.
Fig. 35—Showing how photo-electric stopping potentials of different metals are compared by rotatingandin vacuointo the position of.
Fig. 35—Showing how photo-electric stopping potentials of different metals are compared by rotatingandin vacuointo the position of.
But a beautiful discovery by Klein and Rosseland[187]a little later, in Bohr’s Institute, made this conclusion unnecessary. For it showed that there was an intermediate process, namely, a so-calledcollision of the second kind, by means of which the energymight be transferred without loss,indirectlyfrom the light-wave to the conduction electron, thus obviating the necessity of adirecttransfer. In other words, the Klein and Rosseland discovery proved that the energycould be transferred from the light-wave to the conduction electron by being absorbed first byan atom, which would thus be changed from the normal to the excited state, i.e., the state in which one of its electrons has been lifted from a normal to an outer orbit. This excited atom could then return to its normal statewithout radiationby a collision “of the second kind,” which consists in transferring its whole absorbed energyto a free or conduction electron. The reality of this phenomenon has been experimentally checked by Franck and Cario.[188]This important discovery then left the evidence for localized light-quanta precisely where it was before.[189]
Within the past year, however, a young American physicist, Dr. A. H. Compton, of the University of Chicago, has discovered another new phenomenon which constitutes perhaps the best evidence yet found in favor of Einstein’s hypothesis of localized light-quanta.
Compton’s procedure is as follows. Assuming, for the sake of obtaining quantitative relations, the correctness of Einstein’s hypothesis, he argues that when such a “light-quanta” collides with afreeelectron the impact should be governed by the laws which hold for the collision between any material bodies. These are two in number, namely: (1) the principle of the conservation of energy; (2) the principle of the conservation of momentum (Newton’s Third Law).
Now the energy of a light-quanta, as heretofore shown, is.It moves with the speed of light,,and if its momentum is taken as,it follows at once from the Einstein relativity relationbetween energy and mass, namely,,that its momentum is.This is seen by substituting in the foregoing Einstein relationfor energy. Or, if preferred, the same expression for momentum may be deduced easily from the established laws of light-pressure.
The qualitative results of the preceding assumptions are immediately seen to be as follows. The light-quanta, by colliding with the free electron necessarily transfers some of its energy to it, and therefore, if it arrives with the energy,it must recoil from the impact at some anglewith a smaller energy,and therefore a lower frequency,than that with which it impinged. In other words,light waves should be changed from a higher frequency to a lower—from blue toward red—by impact with a free electron.
A second qualitative result is that, since the mass of the light-quanta, as defined above, is even for the hardest-rays (), of the order of a tenth of the mass of the electron, it is impossible from the laws of elastic impact that it transfer more than a small part of its energy to it. In other words,if Compton’s assumptions are correct, the photo-electric effect, in which there certainly is such a complete transfer, cannot possibly represent the interaction between a light-wave and a free electron. When the electron isboundin the atom there is no difficulty of this sort, for the huge mass of the atom then permits the momentum equation to be satisfied without forbidding the practically complete transfer of the energy to one of its electrons. From this point of view, then, the photo-electric effect represents the interaction between ether-waves andboundelectrons—the Compton effect the interaction between ether-waves andfreeelectrons.
The quantitative results which can be deduced from Compton’s assumptions are definite and simple. Combining the energy and momentum equations in the manner shown inAppendix Hhe obtains easily the resultin whichrepresents the increase in wave-length due to the “scattering” of the incident beam by free electrons, andis the angle between the original direction of the beam and the direction at which the scattered waves come to the measuring apparatus.
Compton then tested this relation experimentally,[190]using as his incident waves the characteristic-rays from a molybdenum target, and as his scattering substance the free (or substantially free) electrons found in graphite.He found indeed that the-line of molybdenum was shifted toward longer wave-lengths just as predicted, and in approximately the correct amount. There was also an unshifted line presumably due to scattering by bound electrons.
Compton had used an ionization-chamber spectrometer for locating his lines. Ross[191]repeated these experiments at Stanford University, California, using the more accurate photographic plate for locating his lines, but still using graphite as the scattering substance. His published photograph shows a line shifted the correct amount and also an unshifted one, but he commented on the fact that the shifted line shows no sign of a separation of theandcomponents while they are clearly separate in the direct picture.
Duane and his collaborators repeated the Compton experiments at Harvard, using again the ionization chamber method, and failed to obtain any trace of the Compton shift. At the February meeting of the Physical Society, 1924, they took the view that the Compton effect did not exist, but that what both Compton and Ross had observed was the-rays of molybdenum with their energy diminished by the work necessary to eject electrons from theshell of the carbon atom.[192]This would actually produce a “scattered line” from carbon which would be practically coincident with Ross’s published line, though it should not give a dependence ofupon anglesuch as Compton had observed.
A few weeks before the date of this writing, at the Norman Bridge Laboratory of Physics at Pasadena, Becker, Watson, and Smythe,[193]using aluminum as a scatterer, obtained a Compton-effect photograph which showed both components of the-rays of molybdenum displaced by an amount which could be measured with an accuracy of about 1 per cent (as checked by the author) and within this limitthe agreement with the displacement computed by the foregoing Compton equation was exact. In this case the Duane-effect-line is completely removed from the Compton-effect-position, and it too was found upon the photographic plate.This furnishes, I think, unambiguous evidence for the reality of the Compton effect. Ross also informs me that he has obtained the Compton shifted line from a number of other elements besides carbon—elements in which the Duane effect could not possibly be confused with it.
The accompanying plate sinews inFig. 36one of the Becker, Watson, and Smythe recent photographs. This one was not taken with sufficient resolution to show the-line as a doublet, but is more reproducible than the one that did. The direct images of both the- and-lines of molybdenum are shown, labeledand,and, a short distance to the right of each, appears the displaced Compton-shifted-line markedand.
At the moment, then, Einstein’s hypothesis of localized light-quanta is having new and remarkable successes. Duane,[194]Epstein,[195]and Ehrenfest[196]have perhaps made some slight advances also in the direction of accounting for interference in terms of it. But the theory is as yet woefully incomplete and hazy. About all that we can say now is that we seem to be driven by newly discovered relations in the field of radiation to the hypothetical use of a fascinating conception which we cannot as yet reconcile at all with well-established wave-phenomena.
To be living in a period which faces such a complete reconstruction of our notions as to the way in which ether waves are absorbed and emitted by matter is an inspiring prospect. The atomic and electronic worlds have revealed themselves with beautiful definiteness and wonderful consistency to the eye of the modern physicist, but their relation to the world of ether waves is still to him a profound mystery for which the coming generation has the incomparable opportunity of finding a solution.
In conclusion there is given a summary of the most important physical constants the values of which it has become possible to fix,[197]within about the limits indicated, through the isolation and measurement of the electron.
i036Fig. 36—THE COMPTON EFFECTThe photograph shows the change of wave-length of ether-waves, from blue toward red, because of scattering by free electrons,andare the initial characteristic-ray lines of molybdenum,andthese same lines after suffering scattering in aluminum.
Fig. 36—THE COMPTON EFFECTThe photograph shows the change of wave-length of ether-waves, from blue toward red, because of scattering by free electrons,andare the initial characteristic-ray lines of molybdenum,andthese same lines after suffering scattering in aluminum.
Fig. 36—THE COMPTON EFFECT
The photograph shows the change of wave-length of ether-waves, from blue toward red, because of scattering by free electrons,andare the initial characteristic-ray lines of molybdenum,andthese same lines after suffering scattering in aluminum.
i037Fig. 37—FINE STRUCTURE OF SPECTRAL LINES IN THE EXTREME ULTRA-VIOLETThe photograph shows the character of the resolution obtained in the recent study by Bowen and Millikan of the fine structure of spectral lines in the extreme ultra-violet. The seven lines in brackets on the left are components of the 834.0 oxygen line. Their total separation is but about two angstroms. The bracketed doublet on the right is one of the many studied, the separation of which is predicted by the theoretical-relativity-doublet formula.
Fig. 37—FINE STRUCTURE OF SPECTRAL LINES IN THE EXTREME ULTRA-VIOLETThe photograph shows the character of the resolution obtained in the recent study by Bowen and Millikan of the fine structure of spectral lines in the extreme ultra-violet. The seven lines in brackets on the left are components of the 834.0 oxygen line. Their total separation is but about two angstroms. The bracketed doublet on the right is one of the many studied, the separation of which is predicted by the theoretical-relativity-doublet formula.
Fig. 37—FINE STRUCTURE OF SPECTRAL LINES IN THE EXTREME ULTRA-VIOLET
The photograph shows the character of the resolution obtained in the recent study by Bowen and Millikan of the fine structure of spectral lines in the extreme ultra-violet. The seven lines in brackets on the left are components of the 834.0 oxygen line. Their total separation is but about two angstroms. The bracketed doublet on the right is one of the many studied, the separation of which is predicted by the theoretical-relativity-doublet formula.