The diffraction of light by metallic screens

Abstract

Gouy discovered that when a metallic screen with a sharp and highly-polished edge is held in the path of a pencil of light, its boundary appears as a luminous line diffracting light through large angles, both into the region of shadow (interior diffraction) and into the region of light (exterior diffraction). He noticed further that this diffracted light is strongly polarised, but in perpendicular planes in the two regions mentioned; the colour of the diffracted light and its state of polarisation depend in a remarkable manner on the material of the screen and on the extent to which its edge is rounded off in the process of polishing. When the edge is viewed through a double-image-prism from within the shadow, only that image appears coloured which is more intense and is polarised with the magnetic vector parallel to the edge. The second image which is fainter and is polarised with the electric vector parallel to the edge, appears perfectly white. When the incident light is polarised in any arbitrary azimuth, the diffracted light is found to exhibit elliptic polarisation. These and other results have been confirmed by later observers. Gouy’s experimental results were discussed by Poincaré on the basis of the electromagnetic theory of light in two memoirs published in the “Acta Mathematica.” The special case of an ideal screen (plane or wedge-shaped), supposed perfectly-reflecting and having a sharp edge, is amenable to complete theoretical treatment, and was dealt with by Poincaré himself, and later in a rigorous manner by Sommerfeld, and following him by numerous other mathematicians. The behaviour of actual metallic screens, however, differs considerably from that found theoretically for this ideal case. Though attempts have been made by Poincaré himself in the memoirs quoted, and later also by Epstein, to take the nature of the screen and the rounding of its edge into account, it cannot be said that Gouy’s observations have so far received a complete or satisfactory explanation. We propose in this paper to discuss more particularly the influence of the material of the screen on the diffraction by a sharp edge, and to show how it may be explained in a very simple manner. The case of rounded edges is reserved for discussion in a separate paper.