© Daniel Kleppner.
If two slits are spaced far enough apart (left), no interference pattern appears on the screen. If the two slits are close together (center), the interference pattern we've seen throughout this unit appears. However, if the slits are placed an intermediate distance apart (right), there is a region where the diffraction patterns overlap, and a region in which they do not overlap. In this third case, you can observe either the wave nature or the particle nature of light, depending on where you place your detector. (Unit: 5)