The fundamental unit of electromagnetic phenomena is the photon, the smallest possible amount of electromagnetic energy of a particular wavelength. Photons, which are without mass, move at the speed of light—300,000 km/sec (186,000 miles/sec) in the form of waves analogous to the way waves propagate through the oceans. The energy of a photon determines the frequency (and wavelength) of light that is associated with it. The greater the energy of the photon, the greater the frequency of light and vice versa.
The entire array of electromagnetic waves comprises the electromagnetic (EM) spectrum. The waves are called electromagnetic because they consist of combined electric and magnetic waves that result when a charged particle (electron) accelerates. The EM spectrum has been arbitrarily divided into regions or intervals to which descriptive names have been applied. At the very energetic (high frequency; short wavelength) end are gamma rays and x-rays. Radiation in the ultraviolet region extends from about 1 nanometer to about 0.36 micrometers. It is convenient to measure the mid-regions of the spectrum in these two units: micrometers (µm), a unit of length equivalent to one-millionth of a meter, or nanometers (nm), a unit of length equivalent to one-billionith of a meter. The visible region occupies the range between 0.4 and 0.7 µm, or its equivalents of 400 to 700 nm. The infrared (IR) region, spans between 0.7 and 100 µm. At shorter wavelengths (near .7 µm) infrared radiation can be detected by special film, while at longer wavelengths it is felt as heat.
Longer wavelength intervals are measured in units ranging from millimeters (mm) through meters (m). The microwave region spreads across 1 mm to 1 m; this includes all of the intervals used by man-made radar systems, which generate their own active radiation directed towards (and reflected from) targets of interest. The lowest frequency (longest wavelength) region—beyond 1 m—is associated with radio waves.