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-called collision of the second kind, by means of which the energy might be transferred without loss, indirectly from the light-wave to the conduction electron, thus obviating the necessity of a direct transfer. In other words, the Klein and Rosseland discovery proved that the energy could be transferred from the light-wave to the conduction electron by being absorbed first by [Pg 256] an 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 state without radiation by a collision “of the second kind,” which consists in transferring its whole absorbed energy to 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 a free electron 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 relation [Pg 257] between energy and mass, namely, , that its momentum is . This is seen by substituting in the foregoing Einstein relation for 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 angle with 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. [Pg 258]