Diffraction-order feasibility
= Diffraction-order feasibility
A formal grating order must have a real outgoing angle. For <normal incidence>, $|m|\lambda\le d$ is necessary. Oblique incidence can access orders outside that normal-incidence range; reflection Littrow geometry has $m\lambda=2d\sin\theta$. The ideal <resolving power of a diffraction grating> applies only to an order that exists and has sufficient signal.