Light is one kind of electromagnetic energy. There are many other types, including heat energy and
radio energy. The only difference between the various types of electromagnetic energy is the frequency of
their waves (rate of vibration). The term SPECTRUM is used to designate the entire range of
electromagnetic waves arranged in order of their frequencies. The VISIBLE SPECTRUM contains only
those waves which stimulate the sense of sight. You, as a technician, might be expected to maintain
equipment that uses electromagnetic waves within, above, and below the visible spectrum.
There are neither sharp dividing lines nor gaps in the ELECTROMAGNETIC SPECTRUM. Figure
1-22 illustrates how portions of the electromagnetic spectrum overlap. Notice that only a small portion of
the electromagnetic spectrum contains visible waves, or light, which can be seen by the human eye.
Figure 1-22.Electromagnetic spectrum.
In general, the same principles and properties of light waves apply to the communications
electromagnetic waves you are about to study. The electromagnetic field is used to transfer energy (as
communications) from point to point. We will introduce the basic ANTENNA as the propagation source
of these electromagnetic waves.