A Fabry-Perot interferometer is used to separate the spectral lines of hydrogen from those of deuterium. If another spectral line is present, a series of independent darker and lighter concentric rings can be seen. The appearance of these rings depends upon the separation of the elements of the interferometer, i.e., the thickness of the air film between the surfaces.
Appropriate data can then yield the separation of the spectral wavelengths.
The recommended experimental procedure below offers some helpful steps in the process. Carefully align the mirrors with the aid of a helium-neon laser. Measure the separation of the sodium doublet . Sodium doublet measurement. Align the mirrors with use of He-Ne laser.