Plane-parallel quartz glass plate

A practical realization of the multiple beam-interference discussed in this section may use either a solid plane-parallel glass or fused quartz plate with two coated reflecting surfaces (Fabry-Perot etalon. Fig. 4.41a) or two separate plates, where one... 

Plane parallel plates of glass or fused quartz with reflecting surfaces are called etalons. In laser spectroscopy they are mainly used as wavelength-selective transmission filters within the laser resonator to narrow the laser bandwidth (see Sect.6.4). The wavelength or frequency for the transmission maximum of m order, where the optical path between successive beams is as = mx, can be deduced from (4.56b) and Fig.4.35 to be... 

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