With incoherent sources, light is emitted by individual atoms or molecules, and the resulting beam is the summation of countless individual events, each of which lasts on the order of 10-8 s. Thus, a beam of radiation from this type of source is not continuous but instead is composed of a series of wave trains that are a few meters in length at most. Because the processes that produce trains are random, the phase differences among the trains must also be variable. A wave train from slit B may arrive at a point on the screen in phase with a wave train from c so that constructive interference occurs; an instant later, the trains may be totally out of phase at the same point, and destructive interference occurs. Thus, the radiation at all points on the screen is governed by the random phase variations among the wave trains; uniform illumination, which represents an average for the trains, is the result.