A bunch of metal plates, each with either one or two small slits cut in them, have been mixed up, and you have been charged with sorting them out. You have a laser of unknown wavelength which you shine through the slit or slits in each of the plates, observing the diffraction pattern the plate causes to appear on a screen 1 m away.
A plate produces a diffraction pattern with a central bright maximum, and minima at 3, 6, and 9 cm from the center of this maximum. Is this a double-slit plate or a single-slit plate?