Propagation and Absorption of EMR in The Earth's Atmosphere
Most electromagnetic waves of higher frequency than visible light (UV and X-rays) are blocked by absorption from electronic excitation in ozone and dioxygen (for UV), and by ionization of air for energies in the extreme UV and above. Visible light is well transmitted in air, as it is not energetic enough to excite oxygen, but too energetic to excite molecular vibrational frequencies of molecules in air.
Below visible light, a number of absorption bands in the infrared are due to modes of vibrational excitation in water vapor. However, at energies too low to excite water vapor the atmosphere becomes transparent again, allowing free transmission of most microwave and radio waves.
Finally, at radio wavelengths longer than 10 meters or so (about 30 MHz), the air in the lower atmosphere remains transparent to radio, but plasma in certain layers of the ionosphere of upper Earth atmosphere begins to interact with radio waves (see skywave). This property allows some longer wavelengths (100 meters or 3 MHz) to be reflected and results in farther shortwave radio than can be obtained by line-of-sight. However, certain ionospheric effects begin to block incoming radiowaves from space, when their frequency is less than about 10 MHz (wavelength longer than about 30 meters).
Read more about this topic: Electromagnetic Radiation
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