Diffraction by a black half-plane: theory and observation

Abstract
It is clear what properties should be possessed by an ideal black screen that is plane, infinite and thin, namely, no reflection and no transmission of an incident electromagnetic wave. If an aperture is cut in this infinite screen, however, it is by no means clear what diffracted field should be expected, there being no compelling local definition of blackness, but only rival theoretical models. Such a screen is realizable approximately by using a highly absorbing material, and blackened screens are present, for example, in most optical instruments. To explore the region very close to a black screen it is more practical to use microwaves. Here, for a half-plane screen geometry, we present measurements of the field closer to the diffracting edge than a few wavelengths, made by using 30 mm microwaves and the modulated scatterer technique, and we compare them with two theoretical models, namely, Kirchhoff diffraction and Sommerfeld black half-plane diffraction. The fields from both models are rederived in unconventional ways, and both are found to be in good general agreement with the measured field on the side of the screen away from the source, but the Sommerfeld field matches the experiment much better on the source side. It is plausible that such differences as remain are due to the imperfect absorption of the screen and its finite thickness.

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