Origin and sources of electromagnetic waves
The whole range of electromagnetic radiation pours on to the earth from the sun and other heavenly bodies in outer space. Those frequencies which are stopped by the earth’s atmosphere have been detected by instruments in man-made satellites. Otherwise some of the main sources on earth are given in the table on page 308.
Fig. 26.21. Skylab If. A picture taken against a black sky background from the
service module, just after the satellite was launched in May 1973. This 77 t
spacecraft sent valuable information to earth over a period of six years before its final disintegration in July 1979. Its power requirements came from the solar panels which transferred energy in the sun’s radiation to energy in an electric current. Geostationary satellites which orbit the earth at a height of about 35 000 km and maintain the same position above its surface are used for communication purposes, mainly telephone and television links. Weather satellites transmit cloud cover pictures to earth for use in weather forecasting