When we send an electromagnetic short wave to the sky, it reflects due to the ionosphere effects. But if we send it horizontally, is it correct that it moves around the surface of the earth, and if it has enough energy, it can return to its first position?
If yes, then how could that happen?
Answer
Due to the refraction index dependence on the air density, optical (and radio) rays bend in the atmosphere. It turns out that an atmospheric layer with temperature inversion (temperature increasing with height) can create conditions where the curvature of the ray would match the curvature of the Earth surface. This is called "circulating rays" and these web-pages contain explanations and references on this phenomenon: http://mintaka.sdsu.edu/GF/explain/simulations/ducting/duct_intro.html, http://mintaka.sdsu.edu/GF/explain/atmos_refr/bending.html#circulating
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