A Fabry-Pérot etalon for optical filtering and generation of interference rings in the normal Zeeman effect experiment.
The fixed etalon consists of a substrate with a double-sided, partially reflective mirror coating of high reflectivity. The substrate and mirrors form an optical resonator which fulfils the resonance conditions of the red cadmium line (λ = 644 nm).
The tilt of the etalon to the optical axis can be adjusted with three adjustment screws in the housing, allowing the displayed pattern of interference rings to be shifted horizontally and vertically.