Electromagnetic radiation sounds like dangerous stuff—and, in fact, some of it is. But that the word radiation need not set off sirens in your head. It just describes any way energy is transmitted from one place to another without the need for a physical connection between the two places. We use it as a general term to describe any form of light. It is important that radiation can travel without any physical connection, because space is essentially a vacuum; that is, much of it is empty. If you went on a space walk clicking a pair of castanets, no one, including you, would hear your little concert. Sound is transmitted in waves, but not as radiation. Sound waves require some medium to travel in. So despite what most science fiction movies would lead you to believe, explosions in space are silent. Light (and other forms of electromagnetic radiation) requires no such medium to travel, although many physicists tried in vain to detect a medium, which they called the ether. We'll talk more about this fact in a moment.
The electromagnetic part of the phrase denotes the fact that the energy is conveyed in the form of fluctuating electric and magnetic fields. These fields require no medium to support or sustain them.
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