The Quantum Theory - Softcover

Softcover

ISBN 10: 1549763997 ISBN 13: 9781549763991

THE old saying that small causes give rise to great effects has been confirmed more than once in the history of physics. For, very frequently, inconspicuous differences be- tween theory and experiment (which did not, however, escape the vigilant eye of the investigator) have become starting- points of new and important researches. Out of the well-known Michelson-Morley experiment, which, in spite of the application of the most powerful methods of exact optical measurement, failed to show an influence of the earth's movement on the propagation of light as was predicted by classical theory, there arose the great structure of Einstein's Theory of Eelativity. In the same way the trifling difference between the measured and calculated values of black-body radiation gave rise to the Quantum Theory which, formulated by Max Planck, was destined to revolutionise in the course of time almost all departments of physics. The quantum theory is yet comparatively young. It is therefore not surprising that we are confronted with an unfinished theory still in process of development which, changing constantly in many directions, must often destroy what it has built up a short time before. But under such circumstances as these, in which the theory is continually deriving new nourishment from a fresh stream of ideas and suggestions, there is a peculiar fascination in attempting to review the life-history of the quantum theory to the present time and in disclosing the kernel which will certainly out- last changes of form, 1 CHAPTER I The Origin of the Quantum Hypothesis � I. Black-Body Radiation and its Realisation in Practice THE Quantum Theory &st saw light in 1900. When, in the years immediately preceding (1897-1899), Lummer and Pringsheim made their fundamental measurements ' of black-body radiation at the Beichsanstalt, they could ba.ve had no premonition that their careful experiments would become the starting-point of a revolution such as has seldom occurred in physics. In the field of heat radiation chief interest at that time was centred in the radiation of a black body (briefly called " black- body " radiation), that is, of a body which absorbs completely all radiation which falls on it and which thus reflects, trans- mits, and scatters 2 none. We may shortly call to mind the following facts. It is known that any body at a given temperature sends out energy in the form of radiation into the surrounding space. This radiation is not energy in a single simple form but is made up of a number of single radiations of different colours, i.e. of different wave-lengths X or of different frequencies s v. In other words, it forms in general a spectrum in which radiations of all frequencies between v = and v = oo are represented. Further, these radiations are present in varying " intensities." We define this term thus. Consider the radiation emitted from unit surface of the body per second in a certain direction ; break it up spectrally and cut out of the spectrum a small frequency interval dv such that it contains all frequencies between v and V + dv. The energy of radiation E^ thus sliced out (namely, the emissivity of the body for the frequency v)

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