First minimum of a finite slit array
= First minimum of a finite slit array
For $N$ equally spaced, coherently illuminated narrow slits at normal incidence, the <phasor> sum is proportional to $\sin(N\delta/2)/\sin(\delta/2)$, where $\delta=2\pi d\sin\theta/\lambda$. The first zero around the central maximum has $d\sin\theta=\lambda/N$. Principal maxima are removable zero-over-zero limits, not <intensity> zeros.