Atmospheric Absorption of Radiation

Although the stars emit radiation of all wavelengths, most regions of the spectrum cannot be observed from the earth’s surface because of absorption by our atmosphere. All parts of the spectrum are affected to some extent, but for some regions of wavelength the atmosphere is completely opaque, even when observations are made from high mountain tops. Two regions of the electromagnetic spectrum to which the atmosphere is particularly transparent are the ‘visible’ region and a ‘radio’ region with wavelengths between 1 cm and 100 m. These two transparent regions are often referred to as the optical window and the radio window respectively. The extent of atmospheric absorption across the electromagnetic spectrum is illustrated in figure 3.1.