The Change of Phase on Reflection of Light from the Silvered Interfaces of an Etalon

In making wavelength determinations to the accuracy of one-thousandth of an Angstrom with a Fabry and Perot Etalon it is necessary to know the wavelength of the line to one-tenth of an Angstrom, the thickness of the etalon and the change of phase on reflection of the light from the interfaces of the etalon. This change of phase is a function of the wavelength, varies with the metal, the way it is deposited and with the surface upon which it is deposited. A separate determination must be made for each etalon. This change of phase is usually expressed as a correction applied to the wave length determination