Each section of a wave train has a dominant frequency that changes randomly from one section to the next. The color of each section corresponds to its dominant frequency—eg, in A, the red section is lower in frequency, and the blue section is higher in frequency. The length of each section is the coherence length. A, Broadband wave trains have a short coherence length. B, Compared with A, the light represented in B has a narrower bandwidth and consequently a longer coherence length,