THE ELECTROMAGNETIC SPECTRUM
For electrical engineers the word electromagnetics typically conjures up
thoughts of antennas, transmission lines, and radio waves, or maybe
boring lectures and “all-nighters” studying for exams. However, this
electrical word also describes a broad range of phenomena in addition
to electronics, ranging from X-rays to optics to thermal radiation. In
physics courses, we are taught that all these phenomena concern electromagnetic
waves. Even many nontechnical people are familiar with
this concept and with the electromagnetic spectrum, which spans from
electronics and radio frequencies through infrared, visible light, and
then on to ultraviolet and X-rays. We are told that these waves are all
the same except for frequency. However, most engineers find that even
after taking many physics and engineering courses, it is still difficult to
see much commonality across the electromagnetic spectrum other than
the fact that all are waves and are governed by the same mathematics
(Maxwell’s equations). Why is visible light so different from radio
waves? I certainly have never encountered electrical circuits or antennas
for visible light. The idea seems absurd. Conversely, I have never
seen FM radio or TV band lenses for sale. So why do light waves and
radio waves behave so differently?
Of course the short answer is that it all depends on frequency, but on
its own this statement is of little utility. Here is an analogy. From basic
chemistry, we all know that all matter is made of atoms, and that atoms
contain a nucleus of protons and neutrons with orbiting electrons. The
characteristics of each element just depend on how many protons the
atom has. Although this statement is illuminating, just knowing the
number of protons in an atom doesn’t provide much more than a framework
for learning about chemistry. Continuing this analogy, the electromagnetic
spectrum as shown in Figure 1.1 provides a basic framework for
understanding electromagnetic waves, but there is a lot more to learn.
To truly understand electromagnetics, it is important to view different
problems in different ways. For any given frequency of a wave, there
is also a corresponding wavelength, time period, and quantum of
energy. Their definitions are given below, with their corresponding relationships
in free space.