5 0 D O C U M E N T 2 9 O P T I C A L E X P E R I M E N T 4) The process of emission is, in energetic terms, a directed process. The quan- tum theoretical derivation of Planck’s radiation law requires that, upon emission of a quantum, a momentum of the quantity be transferred onto the emitting atom this means, according to the law of radiation pressure, that the entire energy hv of the quantum emission is given off in one and the same direction. (A. Einstein. Zur Quantentheorie der Strahlung. Phys. Zeitschr. 1917. pp. 121 to 128).[8] These results would necessarily lead to a pure emission theory (corpuscular the- ory) of light if the interference effects did not persistently pose insurmountable ob- stacles to an interpretation from this standpoint. In particular, it seems impossible according to any emission theory that one source of light would radiate coherent light in different directions, which is without a doubt the case the effectiveness of a microscope, for instance, is based on that the light emitted from the object, which is sent to various opposing edges of a microscope lens, be subjected to interfer- ence. Although a corpuscular theory of light cannot do justice to some of the phenom- ena, it does express features of the phenomena of light that find no place in one is accustomed to interpreting with the aid of the undulatory theory. E.g., it likewise enables an understanding of Doppler’s principle. If a moving atom molecule emits a quantum of eigenfrequency (seen from the molecule) in the a direction that forms the angle ϑ from the direction of motion, then the loss in velocity that the molecule experiences from it is . This velocity loss corresponds to the energy loss . As this energy must change into the radiant energy of the quantum, it becomes, altogether, or , which according to the quantum rule must equal hν if ν means the frequency of the quantum from the frame of reference not moving along with it. From this follows equation (1). One thing is of special importance to us. According to the corpuscular theory, one cannot see from a quantum moving through space whether it originates from a moving or stationary molecule, at least if one imagines the quantum as a point mov- ing at the velocity c that is fully characterized merely by an energy value and per- haps a direction of polarization. The quantum could just as well have originated from a molecule at rest of suitable emission frequency. Conversely, we saw that according to the undulatory theory, the elementary process is endowed with formal hv c ----- - [p. 4] ν0 hν0 c -------- cosϑ m ----------- - ⋅ mq hν0cosϑ cm -------------------- - ⋅ hν0--------------- qcosϑ c = hν0 hν0--------------- qcosϑ c + hν0 1 q c --cosϑ - +