The frequency dependence of the reflection coefficient of MgF_2 crystal in the frequency range of 200-800 cm~(-1) at different orientations of the optical axis has been investigated. The experimental data are compared with the calculation results. This comparison confirms that the wave vectors for the extraordinary wave have an open surface. This makes it possible to focus a divergent beam refracted at a flat boundary ori- ented perpendicularly to the optical crystal axis. The focusing effect of a plane-parallel MgF_2 crystal plate is calculated.
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