Measurements of the Zeeman effect commonly involve the use of a Fabry–Pérot interferometer, with light from a source (placed in a magnetic field) being passed between two mirrors of the interferometer. If δD is the change in mirror separation required to bring the m th-order ring of wavelength λ + Δλ into coincidence with that of wavelength λ, and Δ D brings the (m + 1)th ring of wavelength λ into coincidence with the m th-order ring, then Δ Δ λ λ = λ λ 2 δ δ D 2 D Δ Δ D .