Abstract
FROM experimental work carried out by Fresnel many years ago and a theoretical investigation made by K. Schwarzschild1, it is well known that light transmitted through a narrow slit is more or less polarized. Though conditions are rather different in the case of thermal emission from a narrow filament the idea has occurred to me that polarized emission may be obtained in this way in addition to inclined-surface-layer-effects described by Worthing2and others (Arago effect3). Through the courtesy of Dr. G. Siljeholm of the Lumalampan Aktiebolag in Stockholm I have been able to test recently some filaments no thicker than 4.8µ. The experiments were carried out at the Swedish Solar Observatory in Anacapri, and a polarization as high as 28 per cent has been found in the red. The plane of vibration (electric vector) is perpendicular to the direction of the filament. Considering the fact that the 4.8µ filament had not been polished as in the case of Worthing's measurements, the high value obtained in the red (28 per cent instead of 19 per cent found by Worthing) gives some indication that a ‘slot-aerial’ effect might be present. To some extent this view is supported by measurements of a 6.5µ filament showing already a smaller degree of polarization than a polished broad filament. Furthermore, when examining with a Savart polariscope the interference fringes appearing in highly magnified images, it has been found that the polarization appears to be nearly the same in the central part of the wire as at the limb. Though the resolving power may have been insufficient for separating the regions clearly, it seems hard to explain the observation by means of the Arago effect alone.
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References
Schwarzschild, K., Math. Ann., 55, 177 (1901). Compare also similar effects produced in gratings ( Riecke, Lehrbuch der Physik, 2, 375; 1919).
Worthing, A. G., J. Opt. Soc. Amer., 13, 635 (1926).
Öhman, Y., Polarization Measurements in Astronomy (Boulder, 1949).
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ÖHMAN, Y. Polarized Thermal Emission from Narrow Tungsten Filaments. Nature 192, 254 (1961). https://doi.org/10.1038/192254a0
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DOI: https://doi.org/10.1038/192254a0
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