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The indices of refraction of pure ZnS, ZnSe and ZnTe at 2, 80, and 300°K have been measured in the spectral region from 10 to 100 cm -1 . The indices of refraction can be described by the simple dispersion formula for an undamped harmonic oscillator. The static dielectric constants of these materials are obtained from the best fit parameters using a simple dispersion equation. The average indices of refraction of pure CdS and CdTe at 5°K in the spectral region from 10 to 100 cm -1 have also been measured.

A theory of the surface polaritons inside a spatially dispersive medium, occupying a half space, is developed using the recently formulated macroscopic theory of Agarwal, Pattanayak and Wolf. The dispersion relation i.e., the relation between the frequency and the wave vector parallel to the surface, for surface polaritons is presented. Such surface polaritons have been previously, inadequately treated.

Light modes are excited in thin polyurethane films (freely supported) and LiF films (bounded by a silver film). They are registered by the radiation which is emitted from the rough film surface. The dispersion relation together with the observed position of the s and p modes allows one to determine the refractive index and thickness of the polyurethane film whereas in the case of LiF one needs a small anisotropy of the optical properties (≈ 1%) of the film to fit the observed data. The refractive indices could be determined within an error of about 10 −4 .