There are sources that produce electromagnetic radiation in the form of trains with essentially infinite length and constant frequency. Examples include radio-frequency oscillators, microwave sources, and optical lasers. Various mechanical sources, such as a two pronged vibrating tapper in a water-containing ripple tank, produce a mechanical analog of coherent radiation. When two coherent sources are substituted for slit A in the experiment shown in Figure 6-8a, a regular diffraction pattern is observed.