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atmospheric windowsWavebands of the electromagnetic spectrum that can be transmitted through the Earth's atmosphere without significant absorption or reflection by atmospheric constituents. All spectral regions are at least partially absorbed but there are two nearly transparent ranges: the radio window and the optical window (see illustration). The radio window extends from a wavelength of about a millimeter to about 30 meters (i.e. it includes frequencies from roughly 300 gigahertz to 10 megahertz); it thus allows high-energy radio waves from celestial radio sources to be received by ground-based radio telescopes. Lower energies are reflected by the ionosphere while above 100 GHz molecular absorption increases.
The optical window spans wavelengths of about 300 to 900 nanometers (nm) and thus includes some of the near-ultraviolet and near-infrared regions as well as the whole visible spectrum. In addition there are several narrow-band infrared windows at micrometer (μm) wavelengths. The photometric designations of these windows are J (1.25 μm), H (1.6 μm), K (2.2 μm), L (3.6 μm), M (5.0 μm), N (10.2 μm), and Q (21 μm). There are also small but useable windows at 350 and 460 μm. The atmospheric constituents responsible for the dominant absorptions are water vapor, carbon dioxide, nitrous oxide, and ozone. Observatories studying infrared sources are therefore best sited in very dry or mountainous regions where the effect of overlying water vapor is reduced and/or the atmosphere is thinner.
Beyond the low-wavelength end of the optical window no radiation can penetrate the atmosphere: ultraviolet radiation with wavelengths between about 230 and 300 nm is absorbed by atmospheric ozone; shorter wavelengths down to about 100 nm are blocked by oxygen and nitrogen molecules; the shortest wavelengths are absorbed by atoms of oxygen, nitrogen, etc., in the upper atmosphere. The spectral regions from space rendered inaccessible by atmospheric absorption must be studied by means of instruments carried in satellites, rockets, etc. Even then, high-energy ultraviolet and low-energy X-rays from distant stars can be absorbed by hydrogen and helium in the geocorona.