Black Body Radiation*: Any object at a temperature above absolute zero (0 K or -273 degrees C)--including people-- will radiate EM waves of nearly every type. However, the radiation will be strongest at a particular wavelength. As the temperature decreases, the black body radiation curve flattens out with the peak moving toward lower intensities and longer wavelengths. *The graph is from the Wikipedia.The classical theory curve shown on the graph is now considered obsolete.

Nature's EM Spectrum

The EM spectrum extends all the way from radio waves, which can be over a kilometer long, to gamma rays, with waves less than 1/1000 of a nanometer long. Most EM waves are invisible. Many, including some types of radio waves, microwaves, and gamma rays, can pass through not only air but solid objects like the walls in a typical house. Some are passing through you at this moment!

Whether a material is transparent (lets EM waves through) or opaque (blocks EM waves) is a complex issue. Ozone gas lets visible light through but tends to block ultraviolet. Clouds tend to block visible light but not ultraviolet. Ordinary window glass, a dense solid, is transparent to visible light but opaque to infrared.

Whether a material is or isn't transparent to a given form of EM waves is a very big deal. In fact, it's the cause of global warming. Most EM waves from the Sun pass through Earth's atmosphere unimpeded by carbon dioxide in the air. The waves bring energy from the Sun and warm Earth. Earth re-radiates some of this energy as infra red waves, a process that cools its surface. Unfortunately, carbon dioxide blocks these waves from traveling into outer space. The result: global warming.

The Sun is the single biggest natural source of EM waves on Earth with 50% arriving in the form of visible light. Most of the rest arrives as invisible near-infrared waves. However, the Sun also emits a small amount of almost every type of EM wave, including some waves that can be harmful. For example, the small amount of ultraviolet waves it sends can give us sunburns.

Normally, solar output doesn't interfere with wireless human communications because they mostly occur in the microwave and radio wave parts of the spectrum where there’s little solar output. However, during high levels of sunspot activity, the sun emits higher than normal amounts of EM waves and charged particles (in the form of solar wind). Both can interfere with human use of the EM spectrum.

While the Sun is by far the largest source, it’s not the only celestial object sending us EM waves. EM waves come to us from many sources, including planets like Jupiter and galaxies like the Milky Way. If we limited astronomy only to information arriving via visible light we would be blind to most of the information sent us. Through radio astronomy we open ourselves up to vast amounts of additional information brought to us by the hidden world of the electromagnetic spectrum.

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