= Solution
A $1.5\,\mathrm{mm}$ beam illuminates only $N=300$ slits, giving $mN=900$ and \b[$\delta\lambda_R\simeq0.511\,\mathrm{nm}$]. The two main lobes broaden by $1000/300$ compared with the wider beam. Their $0.5\,\mathrm{nm}$ separation is slightly below the stated Rayleigh threshold: \b[marginally unresolved by that criterion]. This is not an assertion that the profile has only one maximum: the ideal equal-line sum still has two maxima and a modest central dip, as plotted. Inference from real data also requires adequate <signal-to-noise ratio> and a known instrumental profile.
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