EXPERIMENTAL PHASE-CHANGES AT THE MICA SILVER INTERFACE ILLUSTRATE THE EXPERIMENTAL ACCURACY OF THE CENTRAL FILM THICKNESS IN A SYMMETRICAL 3-LAYER INTERFEROMETER

Experimentally measured phase changes of light on reflection at the mica-silver interface are reexamined and found to be in agreement with those calculated using modern optical constants. Phase changes on reflection at a dielectric-silver interface can therefore be calculated using the well-known analytical (of. empirical) expressions and the optical constants, provided the refractive index of the dielectric is known or measured and the silver films are prepared in a similar manner. This discussion is relevant to measurements obtained from the surface forces apparatus. When the surface separation is calculated by Airy's method, we show that the phase changes on reflection at the dielectric-silver interface at the reference wavelengths are either explicitly or implicitly accounted for in all the expressions. We also show that the surface forces technique (spectrometer resolution, similar to 32 Angstrom mm(-1)) is inaccurate for measuring the thickness of very thin aqueous films ( < 10 Angstrom) and that for all practical purposes the central film thickness has to be > 50 Angstrom to achieve a resolution of 1 Angstrom.