Missing diffraction order
= Missing diffraction order
A <missing diffraction order> occurs when a principal maximum of an array factor coincides with a zero of the individual aperture's <diffraction> envelope. For identical slits of width $a$ and centre spacing $D$, order $m$ has $\sin\theta_m=m\lambda/D$. It is missing if $ma/D$ is a nonzero integer. This is cancellation of field contributions, not loss of the incident <wavelength> or a failure of the <grating equation>.