You are designing a lecture demonstration on diffraction gratings. You plan to use light from a laser pointer (λ = 630 nm) and shine the light from the grating on the far wall of the lecture room (W = 9.0 m). You want the separation between the central diffraction spot and one of the spots closest to it on the wall to be 1.0 m apart (Fig. 25.26B). The grating store offers gratings with a specified number of slits per millimeter. How many slits per millimeter do you want for your demonstration?