Refractive-index measurements with a Pellin-Broca prism apparatus.

The absolute phase refractive index n relates the phase velocity v in matter to the vacuum light velocity c(n » civ). It is determined by the molecular polarizability of the medium and gives insight to the interaction of light with matter and the structure of substances. 3 Additionally the group refractive index ng is needed to describe light pulse propagation, 4 5 which is defined as the ratio of vacuum light velocity to group velocity vg in matter (ng = c/vg). Various techniques have been developed to measure the refractive index n (for reviews see Refs. 2,7, and 8). For liquids and solids most methods use the refraction or total reflection of light beams at prisms. The refractive indices of gases are generally measured with interferometers. The spectral range of commercially available refractive-index instruments is often limited to the sodium D line (wavelength = 589.3 nm) or some discrete lines of spectral lamps in the visible. The group refractive index ng may be calculated from the wavelength dependence of the refractive index n(A). Direct measurements of the group refractive index are scarce. In this paper we describe a versatile apparatus for determining the phase refractive indices of liquids and solids. The wavelength may be continuously varied. Measuring phase refractive indices at closely spaced frequencies allows accurate calculation of the group refractive index. Various light sources such as tungsten halogen lamps, high-pressure xenon or mercury arc lamps, spectral lamps, or lasers may be used. A gen-