REFRACTIVE INDEX OF He4: LIQUID

Changes in the phase refractive index n with temperature have been measured between 1.6 and 4.2° K at λ = 5462.27 Å, for liquid He4 at its saturated vapor pressure, using a metal optical cryostat and a Jamin interferometer. A novel adaptation of the Jamin interferometer has been made so that, in addition, an absolute determination of the group refractive index, nG, could be made using white light of "effective wavelength" 5595 ± 40 Å. When the dispersion correction is made, the phase index for the Hg green line at T55E = 3.700° K is n = 1.026124 ± 0.000035. The relative measurements have been adjusted to this value. The more than 200 experimental points show a random scatter of less than 5 × 10−6 in index. Using Kerr's density data the polarizability is thus (N0α) = (0.12454 ± 0.00021) cm3 mole−1 at λ = 5462.27 Å, for liquid He4 at 3.7 °K. Within experimental error (N0α) is found to be independent of temperature. Thus the refractive index data may be considered as a measurement of the liquid density and coefficient of expansion.The region near the λ-point is of special interest. The expansion coefficient determined from the refractive index, βn, may be represented within experimental error by 103βnI = +41.5 + 14.5 log|T−Tλ| for T > Tλ, from about 0.1° above Tλ to within 0.01° of Tλ; and by 103βnII = −1.5 + 14.5 log |T−Tλ| for T < Tλ, from about 0.1° below Tλ to within 0.002° of Tλ. This implies that the density–temperature curve has both a vertical tangent and a point of inflection at the λ-point; and that the maximum in density occurs about 0.001° above the λ-point.