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From http://rst.gsfc.nasa.gov/Intro/Part2_4.html
The specific (named) regions or subdivisions of the electromagnetic spectrum are named and plotted.
Mechanisms for generation of electromagnetic radiation are reviewed. The idea of incoming solar radiation
or irradiance and its interaction (relative absorption) with the atmosphere is reviewed, and the notion of
multispectral remote sensing over different spectral intervals is illustrated with a set of astronomical
examples.
Electromagnetic Spectrum: Distribution of Radiant Energies
As noted on the previous page, electromagnetic radiation (EMR) extends over a wide range of energies and
wavelengths (frequencies). A narrow range of EMR extending from 0.4 to 0.7 µm, the interval detected by
the human eye, is known as the visible region (also referred to as light but physicists often use that term to
include radiation beyond the visible). White light contains a mix of all wavelengths in the visible region. It
was Sir Isaac Newton who first in 1666 carried out an experiment that showed visible light to be a
continuous sequence of wavelengths that represented the different color the eye can see. He passed white
light through a glass prism and got this result:
Use of a prism to disperse visible light into its spectral colors.
The principle supporting this result is that as radiation passes from one medium to another, it is bent
according to a number called the index of refraction. This index is dependent on wavelength, so that the
angle of bending varies systematically from red (longer wavelength; lower frequency) to blue (shorter
wavelength; higher frequency). The process of separating the constituent colors in white light is known as
dispersion. These phenomena apply to radiation of wavelengths outside the visible (e.g., a crystal's atomic
lattice serves as a diffraction device that bends x-rays in different directions).
The distribution of the continuum of all radiant energies can be plotted either as a function of wavelength
or of frequency in a chart known as the electromagnetic (EM) spectrum. Using spectroscopes and other
radiation detection instruments, over the years scientists have arbitrarily divided the EM spectrum into
regions or intervals and applied descriptive names to them.The EM spectrum, plotted here in terms of
wavelengths, is shown here.
The EM Spectrum, with specific wavelength intervals assigned Type terms.
Beneath is a diagram that shows the physical phenomena that give rise to these radiation types and the
instruments (sensors) used to detect the radiation. Although the width of this diagram scales closely to the
width of the spectrum chart above, difficulty in centering this second diagram, type below type, forces
some leftward offset; the narrow pair of vertical lines coincides with the visible range in the upper diagram.
Mechanisms (Phenomenology) of generation of EM radiation within wavelength intervals; instruments
commonly used in detection of radiation within different intervals.
Different names for length units within intervals (specified by Types) of the EM spectrum, all based on the
metric system, have been adopted by physicists as shown in this diagram:
Metric units commonly associated with specific Types of EM Radiation.
Although it is somewhat redundant, we reproduce here still another plot of the EM Spectrum, with added
items that are self-explanatory:
The EM Spectrum, in a diagram produced by Electro Optical Industries, Inc.
At the very energetic (high frequency and short wavelength) end are gamma rays and x-rays (whose
wavelengths are normally measured in angstroms [Å], which in the metric scale are in units of 10-8 cm).
Radiation in the ultraviolet extends from about 300 Å to about 4000 Å. It is convenient to measure the midregions of the spectrum in one of two units: micrometers (µm), which are multiples of 10-6 m or
nanometers (nm), based on 10-9 m. The visible region occupies the range between 0.4 and 0.7 µm, or its
equivalents of 4000 to 7000 Å or 400 to 700 NM The infrared region, spanning between 0.7 and 1000 µm
(or 1 mm), has four subintervals of special interest: (1) reflected IR (0.7 - 3.0 µm), and (2) its film
responsive subset, the photographic IR (0.7 - 0.9 µm); (3) and (4) thermal bands at (3 - 5 µm) and (8 - 14
µm). We measure longer wavelength intervals in units ranging from mm to cm. to meters. The microwave
region spreads across 0.1 to 100 cm, which includes all of the interval used by radar systems. These
systems generate their own active radiation and direct it towards targets of interest. The lowest frequencylongest wavelength region beyond 100 cm is the radio bands, from VHF (very high frequency) to ELF
(extremely low frequency); units applied to this region is often stated as frequencies in units of Hertz (1 Hz
= 1 cycle per second; KHz, MHz and GHz are kilo-, mega-, and giga- Hertz respectively). Within any
region, a collection of continuous wavelengths can be partioned into discrete intervals called bands.
http://rst.gsfc.nasa.gov/Intro/Part2_4.html