Refractive Index Of Gas Research Articles

Three-beam interferometer

The instrument described utilizes a beam dividing system similar to that used in the Jamin interferometer, but employs three interfering beams instead of two. This permits the determination of the fractional fringe order with an extremely high degree of accuracy. It can be used for studying very small changes of refractive index in gases and liquids, and, in a slightly modified form, as an auto-collimating interferometer for the precise measurement of the thickness of evaporated films.

Measurement of refractive indices of air, nitrogen, oxygen, carbon dioxide and water vapour at 3360 Mc/s

A sensitive and accurate method for the measurement of the refractive indices of gases at 3 360 Mc/s is described and critically examined to assess the systematic errors. The method is based on the measurement of the change of the resonant frequency of a cavity resonator when evacuated and when filled with a gas. A frequency modulation technique is used in conjunction with an oscillograph display. The standard deviation in the measurement of changes of the resonant frequency of the high-Q-factor cavity employed, was 77 c/s; this made possible the determination of (n - 1), where n is the refractive index, to four significant figures. This accuracy was achieved with the aid of a crystal-controlled frequency standard. The results obtained for (n - 1) × 106 at 0° C, 760 mm Hg are as follows: Dry carbon-dioxide-free air 288.26 ± 0.2 Nitrogen 294.47 ± 0.2 Oxygen 266.17 ± 0.2 Carbon dioxide 494.59 ± 0.35 Water vapour (at 20° C and 10 mm Hg) 61.31 ± 0.1 The permanent electric moment of the water molecule was calculated to be (1.845 ± 0.001) × 10 -18 e.s.u. The method of measurement is equally suitable for other microwave frequencies.

Some measurements on refractive indices of gases in the microwave region

Synopsis A method is described to measure refractive indices of gases at atmospheric pressure and at a frequency of 25,000 Mc/s. The method is based on the change in resonance frequency of a cavity, first filled with a reference gas (nitrogen) and after that with the gas of which the refractive index is to be determined. However, instead of measuring the frequency change of a fixed cavity, we measure the change in length of the cavity, necessary to keep it tuned in both cases to the frequency of the (6,6) inversion line of ammonia. Subsequently the reading with a nitrogne filling is compared to the one of the evacuated cavity, so that all refractive indices are obtained with respect to vacuo. Results of these measurements for ten gases are given. As the absorption is extremely small and the inaccuracy of our measurements of refractive indices amounts to one or two parts in 106 for these gases, the introduction of a complex refractive index is found to be unnecessary.

Refractive indices of gases at high radio frequencies

Refractive indices of gases at high radio frequencies

Dispersion and Refractive Index of Nitrogen Measured as Functions of Pressure by Displacement Interferometry

The Lorentz-Lorenz relation has never been satisfactorily checked by experiment for pressures above atmospheric, because of a lack of trustworthy data on the refractive indices of pure gases as functions of pressure. Such data would also be particularly valuable in the study of polar molecules on account of Maxwell's law connecting the refractive index with the dielectric constant. Due, however, to the difference between optical and electrical frequencies the refractive index must be reduced to zero frequency before Maxwell's law can be applied. This necessitates a precise knowledge of dispersion which is not available for many pure gases. In the present paper a method of measuring simultaneously the dispersion and refractive index of a gas is discussed. The method is justified on theoretical and experimental grounds and applied in a practical way to the measurement of dispersion and refractive index of nitrogen over a range of pressures and at two different temperatures with a high degree of precision.