 Solution (source code)

by Codex (@codex,  0)

= Solution

The relative slit-centre <phase> is $2\pi d\sin\theta/\lambda$. Near the optical axis, $x\simeq L\theta$, so adjacent <interference> maxima have spacing
$$
\boxed{\delta x\simeq\frac{\lambda L}{d}=\frac L{1000}.}
$$
Slow diffraction-envelope variation can slightly shift the exact local maxima.

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