On the influence of magnetic fields on the polarisation of resonance radiation

Abstract

The emission of polarised light by fluorescent sodium and iodine vapours was discovered by one of the present writers many years ago, and a similar phenomenon was looked for repeatedly, but without success, in the case of the much simpler (presumably) type of emission which occurs when a gas or vapour re-emits without change of wave-length the energy which it abstracts from the incident beam, as when the monochromatic radiation 2536 from a mercury arc traverse as exhausted quartz bulb containing a drop of mercury at room temperature, or the light of a sodium flame falls upon a glass bulb containing highly rarefied sodium vapour. Lord Rayleigh has recently discovered, however, that the resonance radiation of mercury vapour shows fairly strong polarisation, and reported results which apparently indicated that the polarisation becomes stronger as the exciting radiation penetrates to greater and greater depths of the vapour, being practically absent at the point where the radiation enters the bulb or tube. This would mean that polarisation of the light results only when the excitation is, by frequencies, a little out of tune with the natural frequency of the mercury atom, a matter of great theoretical interest. In a report of some work done last year by one of us this last observation of Lord Rayleigh was not confirmed, the polarisation appearing of full strength right up to the entrance window of the exciting beam. A possible explanation of Lord Rayleigh's results will be offered in the present paper in which it will be shown that a very weak magnetic field, properly oriented, destroys completely all traces of polarisation.