An etalon is a device for studying spectra by splitting light into its component wavelengths. The etalon (also called the Fabry-Perot interferometer) consists of parallel, partially silvered glass plates, which are accurately separated so that the air film between them has a uniform thickness. Light undergoes multiple reflection between the surfaces and eventually emerges as shown in Figure 1.
The condition for the waves to be in phase depends on the wavelength of the light, the thickness of the film and the angle at which the ray meets the surface. If light travelling along path B travels exactly one wavelength further than light along path A, the two emerging rays will be in phase so will interfere constructively and the beam will be bright. For each angle, only a single wavelength will satisfy the condition and give a bright beam — so different wavelengths (different colours) emerge at different angles.
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