We have measured the diffraction pattern of the edge of a transparent sheet for plane-polarized microwaves (λ=3.0 cm) for angles of incidence from 0° to 45°. The material was expanded polyurethane (Styrofoam) with index of refraction 1.0150 (that of air being 1.0003). The thickness of the sheet, and thus of the rectangular edge, varied from 1.7 to 13.5 wavelengths. Our initial purpose was to study the diffraction of microwaves by phase objects, as an analog that might indicate general ways of improving the contrast in images of phase objects obtained with an electron microscope and, in particular, ways of explaining electron edge diffraction. The high contrast of as much as 2 to 1 of irradiance of the edge pattern for material of such high transparency (99.5% for thickness of 3.4 wavelengths) was surprising. We have explained the major features of our results on the basis of Huygens’s principle and Fresnel’s equations for reflection and refraction at the surface of the edge of finite thickness.
J. N. Turner, D. F. Parsons, and C. L. Andrews, "Diffraction of electromagnetic waves by transparent edges," J. Opt. Soc. Am. 64, 789-793 (1974)