6B-6 Coherent Radiation
In order to produce a diffraction pattern such as that shown in Figure 6-8a, it is necessary that the electromagnetic waves that travel from slits B and c to any given point on the screen (such as D or E) have sharply defined phase differences that remain entirely constant with time; that is, the radiation from slits B and c must be coherent. The conditions for coherence are that (1) the two sources of radiation must have identical frequencies (or sets of frequencies) and (2) the phase relationships between the two beams must remain constant with time. The necessity for these requirements can be demonstrated by illuminating the two slits in Figure 6-8a with individual tungsten lamps. Under this circumstance, the well-defined light and dark patterns disappear and are replaced by a more or less uniform illumination of the screen. This behavior is a consequence of the incoherent character of filament sources (many other sources of electromagnetic radiation are incoherent as well).