Diffraction on a screw, Photo 3 of 4. A small screw is placed in a beam produced by either a Helium-Neon laser operating at 632.8 nm (red, Photos 1 and 2) or a Helium-Neon laser operating at 543.4 nm (green, Photos 3 and 4). The resulting Fresnel diffraction patterns are observed at the same distance from the screw. Note that the fringe spacing is smaller for the green laser.