DIFFRACTION BY A BLACK HALF-PLANE - THEORY AND OBSERVATION

It is clear what properties should be possessed by an ideal black scre en that is plane, infinite and thin, namely, no reflection and no tran smission 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 real izable approximately by using a highly absorbing material, and blacken ed screens are present, for example, in most optical instruments. To e xplore the region very close to a black screen it is more practical to use microwaves. Here, for a half-plane screen geometry, we present me asurements of the field closer to the diffracting edge than a few wave lengths, made by using 30 mm microwaves and the modulated scatterer te chnique, and we compare them with two theoretical models, namely, Kirc hhoff diffraction and Sommerfeld black half-plane diffraction. The fie lds from both models are rederived in unconventional ways, and both ar e 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 mat ches the experiment much better on the source side. It is plausible th at such differences as remain are due to the imperfect absorption of t he screen and its finite thickness.