11.1.1 Introduction

The electromagnetic (EM) spectrum consists of all forms of EM radiation EM waves (radiant energy) propagating through space, from DC to light to gamma rays. The EM spectrum can be arranged in order of frequency and/or wavelength into a number of regions, usually wide in extent, within which the EM waves have some specified common characteristics, such as characteristics relating to the production or detection of the radiation. A common example is the spectrum of the radiant energy in white light, as dispersed by a prism, to produce a "rainbow" of its constituent colors. Specific frequency ranges are often called bands; several contiguous frequency bands are usually called spectrums; and sub-frequency ranges within a band are sometimes called segments.

The EM spectrum can be displayed as a function of frequency (or wavelength). In air, frequency and wavelength are inversely proportional, f = c/ ? (where c ? 3 10 ⁸ m/s, the speed of light in a vacuum). The MKS unit of frequency is the Hertz and the MKS unit of wavelength is the meter. Frequency is also measured in the following sub-units:

• Kilohertz, 1 kHz = 10 ³ Hz

• Megahertz, 1 MHz = 10 ⁶ Hz

• Gigahertz, 1 GHz = 10 ⁹ Hz

• Terahertz, 1 THz = 10 ¹² Hz

• Petahertz, 1 PHz = 10 ¹⁵ Hz

• Exahertz, 1 EHz = 10 ¹⁸ Hz

Or for very high frequencies, electron volts, 1...