30.0 - Introduction
Radio and television signals, x-rays, microwaves:
Each is a form of electromagnetic radiation. If steam
and internal combustion engines symbolize the
Industrial Revolution, and microprocessors and
memory chips now power the Information Revolution,
it almost seems that we have neglected to recognize
the “Electromagnetic Revolution.” Think about it: Can
you imagine life without television sets or cell
phones? You may long for such a life, or wonder how
people ever survived without these devices!
These examples are from the world of engineered
electromagnetic radiation. Even if you think we might
all prosper without such technologies to entertain us,
do our cooking, carry our messages, and diagnose our illnesses, you would be hard-pressed to survive without light. This form of
electromagnetic radiation brings the Sun’s energy to the Earth, warming the planet and supplying energy to plants, and in turn to creatures like
us that depend on them. There are primitive forms of life that do not depend on the Sun’s energy, but without light there would be no seeing, no
room with a view, no sunsets, and no Rembrandts.
Some of the electromagnetic radiation that reaches your eyes was created mere nanoseconds earlier, like the light from a lamp. Other
electromagnetic radiation is still propagating at its original speed through the cosmos, ten billion years or more after its birth. An example of this
is the microwave background radiation, a pervasive remnant of the creation of the universe that is widely studied by astrophysicists.
Back here on Earth, this chapter covers the fundamental physical theory of electromagnetic radiation. Much of it builds on other topics,
particularly the studies of waves, electric fields and magnetic fields.
Electromagnetic radiation: Rainbows and radios. Sundazzled reflections.
Shadowlamps and lampshadows. Red, white, and blue.30.1 - The electromagnetic spectrum
Electromagnetic spectrum: Electromagnetic
radiation ordered by frequency or wavelength.
Electromagnetic radiation is a traveling wave that consists of electric and magnetic
fields. Before delving into the details of such waves, we will discuss the electromagnetic
spectrum, a system by which the types of electromagnetic radiation are classified.
The illustration of the electromagnetic spectrum above orders electromagnetic waves by
frequency and by wavelength. In the diagram, frequency increases and wavelength
decreases as you move from the left to the right. The chart’s scale is based on powers
of 10. Wavelengths range from more than 100 meters for AM radio signals to as small
as 10í^16 meters for gamma rays.
All electromagnetic waves travel at the same speed in a vacuum. This speed is
designated by the letter cand is called the speed of light. (The letter c comes from
celeritas, the Latin word for speed. It might be more accurate to refer to it as the speed of electromagnetic radiation.) The speed of light in a
vacuum is exactly 299,792,458 m/s, and it is only slightly less in air.
The unvarying nature of this speed has an important implication: The wavelength of electromagnetic radiation is inversely proportional to its
frequency. As you may recall, the speed of a wave equals the product of its frequency and wavelength. This means that if you know the
wavelength of the wave, you can determine its frequency (and vice versa). For instance, an electromagnetic wave with a wavelength of 300
meters, in the middle of the AM radio band, has a frequency of 1×10^6 Hz. This equals 3×10^8 m/s, the speed of light, divided by 300 m. The
frequencies of electromagnetic waves range from less than one megahertz, or 10^6 Hz, for long radio waves to over 10^24 Hz for gamma rays.
We will now review some of the bands of electromagnetic radiation and their manifestations. The lowest frequencies are often utilized for radio
Electromagnetic spectrum
Radio waves in AM, FM bands