Second-order direction
= Second-order direction
The pitch corresponding to $500\,\mathrm{mm^{-1}}$ is $D=2.00\,\mathrm{\mu m}$. The normal-incidence <diffraction grating> equation gives
$$
D\sin\theta=m\lambda,\qquad
\boxed{\theta_2=\arcsin\!\left(\frac{2(400\,\mathrm{nm})}{2.00\,\mathrm{\mu m}}\right)=23.58^\circ}.
$$
This locates the array-factor maximum; the single-slit envelope must also be nonzero for an observable peak.