Francesco Grimaldi had already watched sunlight fan after a narrow slit and called it diffraction. Young’s 1801 Bakerian lecture made the wave picture respectable in Britain — mostly with two slits, but a single opening is enough to see the lobes.
Wider slit, narrower fan; more lobes if you ask the tracer for them. The pattern is not a geometric shadow. Drag the jaws and the screen stops looking like a lamp with a card in front.