Mirages occur when air?s refractive index varies with position as a result of uneven heating. Under such conditions, light undergoes refraction continually and thus follows a curved path. Other examples where a varying refractive index is important include the eye?s lens and Earth?s ionosphere, an electrically conductive layer in the upper atmosphere, where the refractive index for radio waves varies with altitude.

The refractive index in the ionosphere is strongly dependent on radio-wave frequency, approaching 1 for high frequencies. Therefore,a. long-distance communication via the ionosphere is more likely at higher frequencies.b. higher frequencies won?t penetrate as far into the ionosphere.c. higher frequencies are more appropriate for satellite-based communication

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Light path ... C B A D (a) Ionosphere "Radio-wave paths A Earth B (b)