Radio waves, like light waves and all other forms of electromagnetic radiation, normally travel in straight lines. Obviously this does not happen all  the time, because long - distance communication depends on  radio waves traveling  beyond the horizon. How radio waves propagate in  other than straight-line paths  is a complicated subject, but one that  need not be a mystery. This page provides  basic understanding of the  principles of electromagnetic radiation, the structure of the Earth's  atmosphere and solar-terrestrial interactions necessary  for a working  knowledge of radio propagation. More detailed discussions and the   underlying mathematics of radio propagation physics can be found in the  references listed under additional resources.