A spherical Fabry Perot interferometer is used to determine the refractive index of argon to a few hundred bars, with a better accuracy than those obtained in preceding experiments.The first and second refractivity virial coefficients have been determined. Measurements have been made for eight wavelengths in the visible spectrum between 4359 and 6329 Å. A value of the third coefficient is given for the first time.Comparisons are made with preceding published values, and with values of dielectric virial coefficients.

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We measured accurately the refractive index of nitrogen at 25 °C between 40 and 400 bar, and calculated the refractivity R = (n2 − 1)/(n2 + 2)1/ρ. Particular attention is given to the determination of the density ρ, which is the limitation of accuracy on R. We calculate the refractivity virial coefficients AR, BR, and CR, the last one being given for the first time.The results are compared with the published data, the dielectric virial coefficients, and the induced absorption. The Buckingham expression of BR is calculated for N2 and compared with our values.

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In a schlieren detection scheme for photodeformation measurements, the divergence of the probe beam that is induced by the axisymmetric but radially inhomogeneous periodic photothermal displacement of the surface of a sample is transformed into an intensity variation by insertion of an iris in front of the detection photodiode. We present three expressions for the intensity profile of a Gaussian laser beam that is reflected by the inhomogeneous photodeformation of a solid. The first expression proceeds from geometrical optics (or photometry), whereas the second one derives from the use of the well-known ABCD law and the third one from diffraction principles. Comparing these formulations of the schlieren signal with their behavior as a function of different geometrical parameters, we obtain the domain of validity of each expression, and we deduce the advantages of the different formalisms.