 Solution

The slit transmissions have the same magnitude but opposite phase. An ideal narrow-slit representation isEquivalently draw two narrow rectangles at $y=\pm d/2$, one at+1 and the other at−1, and zero elsewhere. A finite rectangle width gives each slit its own diffraction envelope. The original aperture plot shows such rectangles with explicitly illustrative width; the negative value is a field phase reversal, not negative transmitted intensity.