4 8 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 each other, the undulatory theory or the electromagnetic field theory, resp., and quantum theory. The undulatory theory has been to this date indispensable to us for the theoretical interpretation of refraction, diffraction, interference, dispersion as well as for an understanding of the connection between optical and electromagnetic phenomena in the narrower sense. It spans an enormous range of phenomena, which we will call the geometrical field of “true optics.” But it fails for all prob- lems of absorption, emission, and generally for all finer energetic properties of light radiation. It is absolutely incapable of explaining Planck’s formula, the spec- tral laws, the laws of the photoelectric effect, photochemical activity, etc. The quan- tum theory, conversely, proves to be an indispensable guide in the area of conformity with a natural law of energy, but has completely failed until now in “true optics.” Without a doubt physicists now favor the view that quantum theory covers deeper features of physical reality than the undulatory theory and that in all prob- lems [on which applic] accessible to both theories quantum theory has proved to be superior. Since, however, the undulatory theory was is capable of describing the phenomena within the range of true optics with exceptional exactitude, without failing in a single case, the belief still prevails today that we shall manage one day to fuse quantum theory and undulatory theory into a single whole without denying the latter exact validity. Let us now regard the process of light emission from a single gas molecule from the point of view of both theories. According to the undulatory theory, an electron vibrating relative to the molecule generates a system of electromagnetic spherical waves. These spherical waves are concentric if the particle emitting molecule as a whole has the velocity zero, eccentric if the emitting particle molecule has a velocity relative to the system of coordinates. The color of the emitted light radi- ation then is not a constant function of the direction of emission but a continuous one (Doppler’s principle) according to the formula , . . . (1) provided v means the frequency of the emitted radiation, q the molecule’s velocity, and ϑ the angle between it and the considered direction of emission.[5] The particle sends out in various directions coherent radiation of various colors. The distance between opposing planes of the same phase, that is, of wavelength λ, is spatially variable. One could therefore see from the undulatory field, even from a finite part of it, whether it originates from a stationary or a moving molecule—if the undula- tory field was perceptible. This dependence local variability of λ diminishes, for freely propagating waves, with the distance from the molecule but can be main- tained without loss over large distances if one allows the waves to pass through a [p. 2] v v0 1 q c --cosϑ - + =
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