= Why the given slit width removes the chosen order
For a slit of width $a$, the <single-slit diffraction> amplitude is proportional to $\operatorname{sinc}(\pi a\sin\theta/\lambda)$. At a grating maximum, $D\sin\theta=m\lambda$, so the envelope factor is
$$
\operatorname{sinc}\!\left(\frac{\pi ma}{D}\right).
$$
The specified $a=1\,\mathrm{\mu m}=D/2$ gives $ma/D=1$ at $m=2$. Thus
$$
\boxed{\operatorname{sinc}(\pi)=0:\quad\text{the second order is a missing diffraction order}.}
$$
The grating maximum coincides exactly with the first single-slit minimum for each wavelength. The preceding width and detector calculations describe the ideal array factor, but this particular rectangular-aperture grating cannot deliver the assumed second-order signal. Pixel refinement cannot repair the missing optical peak. One must change the <aperture> duty cycle or grating profile, or use an available order and recalculate its resolving <power>.
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