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.

Figure 30.25b shows how continuous refraction in the ionosphere enables long-distance radio communication. Waves launched at angles steeper than u don?t refract enough to return to Earth, so they propagate through the ionosphere and on to space. You can therefore conclude thata. all points between A and B receive stronger signals from A than point B receives.b. points between A and B can?t receive signals from A via the ionosphere.c. the refractive index must become infinite at the maximum altitude of the radio signal.

Transcribed Image Text:

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