Refractive Indices of Water Vapor and Carbon Dioxide at Low Pressure

Abstract

The Fabry-Perot interferometer has been used to obtain the dispersion curves for water vapor and for carbon dioxide in the wave-length range 2500A to 8600A at pressures below 20 mm of mercury and at a temperature of 20°C. The results have been formulated to permit computation of the refractive index of air containing these substances. The presence of water vapor in air used for precision wave-length measurements is undesirable, due to possible formation of an adsorbed film of water on the etalon mirrors. This has been observed for evaporated aluminium mirrors and for evaporated silver mirrors.Temperature variation causes the effective thickness of the common form of Fabry-Perot etalon to vary in much greater degree than is predicted by the expansion coefficient of the spacer material. The explanation of this effect leads to the conclusion that the etalon pressure coefficients will show similar anomalies. It is possible to explain qualitatively the discrepancies between the measurements of the refractive index of air made by Meggers and Peters and those by more recent observers.