SUPERPOSITION OF THE DIFFRACTION FIELDS OF APERTURES - AN EXPERIMENTAL TEST

In Kirchhoff diffraction theory it is assumed that the effects of sepa rate apertures in a screen are superposable. Thus, for example, a two- slit pattern is the (complex) superposition of the two individual sing le-slit patterns. The validity of the assumption has been tested with 10 GHz microwaves (lambda = 30 mm) incident at 45 degrees on two paral lel slits, of width 1/2 lambda, in a conducting screen, their spacing being lambda. The diffraction pattern immediately behind the screen wa s measured by using the optically modulated scatterer technique. With the electric field in the incident wave parallel to the slits there wa s, on average, a 7% discrepancy in amplitude from what would be predic ted by superposition of the effects of the two separate apertures. Wit h the electric field perpendicular to the slits the discrepancy increa sed to 25% in amplitude. The results may be interpreted by saying that the two-slit pattern is indeed the result of interference between two separate single-slit patterns, but their phases and amplitudes are di fferent from what would be expected from simple theory. it is as if th e wave emitted from each slit is affected (by 10 or 30%) by whether th e other slit is open or closed. The difference between the results for the two polarizations is explained by the boundary condition on the s urface of the screen, which tends to suppress communication between th e slits when the polarization of the incident wave is parallel to the slits.