Refractive index of crystals from transmission and reflection measurements: MgO in the far-infrared region

In principle, the real n and imaginary k parts of the complex refractive index of a crystal can be determined by measuring, when possible, its transmission and reflection spectra. We show that it is possible to derive the frequency dependence of n and k in a simple way by using exact expressions for the interference-free transmittance and reflectance to describe low-resolution data. We also derive n and k data starting from one high-resolution spectrum. As an example, we have measured the roomtemperature transmittance and reflectance of MgO in the far-infrared region. Reliable n and k data are then obtained in the frequency region 50-250 cm(-1).