Ellipsometric study of polished glass surfaces

The refractive index nf and thickness df of the "polish layer" on the surface of various kinds of glass polished under standard conditions were determined by ellipsometry. The method of ellipsometry employed was based on the measurement of the principal angle of incidence and the ellipticity of the light reflected at this angle. It was found that nf < nb (nb is the bulk index of glass) with glasses which are susceptible to weathering, and nf >nb with chemically durable glasses such as silica glass, Vycor and Pyrex. It is presumed that there are two leading factors which affect the value of nf: One is the very high local pressures exerted on the glass surface by particles of the polishing agent, which may increase nf through densification of glass, and the other is the leaching action of the water-based slurry, which will decrease nf. The value of nf relative to nb will be determined by the balance between these two factors. These hypotheses have been qualitatively verified by annealing the polish layer, which, if the increase in nf be due to densification, ought to decrease nf, and by leaching the polished surface in the slurry for some period.