= Phase-reversed double-slit diffraction
Two equal rectangular apertures of width $a$, centre spacing $D$, and relative <phase> $\pi$ have <Fraunhofer diffraction> amplitude proportional to $2ia\operatorname{sinc}(qa/2)\sin(qD/2)$, with $q=(2\pi/\lambda)\sin\theta$ and $\operatorname{sinc}z=\sin z/z$. Hence
$$
I(\theta)\propto\sin^2(qD/2)\operatorname{sinc}^2(qa/2).
$$
The <interference> zeros satisfy $D\sin\theta=m\lambda$, including the central dark fringe. The single-aperture envelope vanishes at $a\sin\theta=n\lambda$ for nonzero integer $n$. A uniform phase reversal translates the narrow-slit fringes by half a fringe without changing their spacing.
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