Electromagnetic Spectrum

Electromagnetic fields exist at frequencies from DC (0 Hz) to at least 1020 Hz – that’s at least 20 orders of magnitude!

At DC, electromagnetics consists of two distinct disciplines: electrostatics, concerned with electric fields; and magnetostatics, concerned with magnetic fields.

At higher frequencies, electric and magnetic fields interact to form propagating waves. Waves having frequencies within certain ranges are given names based on how they manifest as physical phenomena. These names are (in order of increasing frequency): radio, infrared (IR), optical (also known as “light”), ultraviolet (UV), X-rays, and gamma rays (

-rays). See Table 1.2.1 and Figure 1.2.1 for frequency ranges and associated wavelengths.

Definition: Electromagnetic Spectrum

The term electromagnetic spectrum refers to the various forms of electromagnetic phenomena that exist over the continuum of frequencies.

The speed (properly known as “phase velocity”) at which electromagnetic fields propagate in free space is given the symbol

, and has the value

. This value is often referred to as the “speed of light.” While it is certainly the speed of light in free space, it is also speed of any electromagnetic wave in free space. Given frequency

, wavelength is given by the expression

Table 1.2.1 shows the free space wavelengths associated with each of the regions of the electromagnetic spectrum.

RegimeFrequency RangeWavelength Range

-Ray

< 0.01 nm
X-Ray10–0.01 nm
Ultraviolet (UV)120–10 nm
Optical700–120 nm
Infrared (IR)1 mm – 700 nm
Radio100 km – 1 mm

This book presents a version of electromagnetic theory that is based on classical physics. This approach works well for most practical problems. However, at very high frequencies, wavelengths become small enough that quantum mechanical effects may be important. This is usually the case in the X-ray band and above. In some applications, these effects become important at frequencies as low as the optical, IR, or radio bands. (A prime example is the photoelectric effect; see “Additional References” below.) Thus, caution is required when applying the classical version of electromagnetic theory presented here, especially at these higher frequencies.

Theory presented in this book is applicable to DC, radio, IR, and optical waves, and to a lesser extent to UV waves, X-rays, and

-rays. Certain phenomena in these frequency ranges – in particular quantum mechanical effects – are not addressed in this book.

The radio portion of the electromagnetic spectrum alone spans 12 orders of magnitude in frequency (and wavelength), and so, not surprisingly, exhibits a broad range of phenomena. This is shown in Figure 1.2.1.

For this reason, the radio spectrum is further subdivided into bands as shown in Table 1.2.2.

BandFrequenciesWavelengthsTypical Applications
EHF30-300 GHz10–1 mm60 GHz WLAN, Point-to-point data links
SHF3–30 GHz10–1 cmTerrestrial & Satellite data links, Radar
UHF300–3000 MHz1–0.1 mTV broadcasting, Cellular, WLAN
VHF30–300 MHz10–1 mFM & TV broadcasting, LMR
HF3–30 MHz100–10 mGlobal terrestrial comm., CB Radio
MF300–3000 kHz1000–100 mAM broadcasting
LF30–300 kHz10–1 kmNavigation, RFID

Also shown in Table 1.2.2 are commonly-used band identification acronyms and some typical applications. Similarly, the optical band is partitioned into the familiar “rainbow” of red through violet, as shown in Figure 1.2.1 and Table 1.2.3. Other portions of the spectrum are sometimes similarly subdivided in certain applications.

BandFrequenciesWavelengths
Violet668–789 THz450–380 nm
Blue606–668 THz495–450 nm
Green526–606 THz570–495 nm
Yellow508–526 THz590–570 nm
Orange484–508 THz620–590 nm
Red400–484 THz750–620 nm