Diffraction patterns can be obtained from random sources, such as tungsten filaments, provided that an arrangement similar to that shown in Figure 6-8a is employed. Here, the very narrow slit A assures that the radiation reaching B and c emanates from the same small region of the source. Under this circumstance, the various wave trains that exit from slits B and c have a constant set of frequencies and phase relationships to one another and are thus coherent. If the slit at A is widened so that a larger part of the source is sampled, the diffraction patern becomes less pronounced because the two beams are only partially coherent. If slit A is made sufficiently wide, the incoherence may become great enough to produce only a constant illumination across the screen.