(a)coherent radiation相干輻射To produce a diffraction(繞射) 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.