What are the Energy Range Definitions for the Various
Types of Electromagnetic Radiation?
Last Update: June 30th, 2008
Astronomers have made observations of electromagnetic radiation from
cosmic sources that cover a range of more than 21 decades in
wavelength (or, equivalently in frequency or energy)!
The wavelength/frequency/energy ranges corresponding to the (far) ultraviolet
(UV), extreme-ultraviolet (EUV), X-ray and gamma-ray bands are a matter of
definition rather than science, and hence are of some debate. Astronomers
who study cosmic sources of high-energy electromagnetic radiation
typically assume them to be roughly as shown below, where they are compared
with the UV, optical, infrared (IR) and radio bands. Notice that astronomers
often use non-standard units rather than MKS units, e.g., wavelength units
such as Angstroms (1 A = 10-8 cm) or microns (1 um =
10-4 cm) and energy units such as the
kilo-electron Volt (1 keV = 1.60 x 10-9 erg) or electron-Volt
(1 eV = 1.60 x 10-12 erg).
The fundamental relation between
energy E (in erg) and frequency f (in Hz) is E = h x f, where h, Planck's
Constant, is 6.6261 x 10-27 in cgs units.
The fundamental relation between
wavelength W (in cm) and frequency f (in Hz) is c = W x f, where c, the speed
of light, is 2.9979 x 10-10 cm s-1.
Band Wavelength Frequency Energy No. of decades
Ionospheric Cutoff 15-30 m 10-20 MHz --- ---
Radio: meter+ > 100 cm <300 MHz --- >3.0
Radio: cm + dm 1-100 cm 0.3-30 GHZ --- 2.0
Radio: mm 0.1-1 cm 30-300 GHz --- 1.0
Sub-mm/THz 0.01-0.1 cm 0.3-3 THz 0.001-0.01 eV 1.0
IR 1-100 um 3-300 THz 0.01-1.2 eV 2.0
Near-IR 7000-10000 A 0.3-0.43 PHz 1.2-1.8 eV 0.15
Visible 3200-7000 A 0.43-0.94 PHz 1.8-3.9 eV 0.3
UV 900-3200 A 0.94-3.33 PHz 3.9-14 eV 0.6
EUV 100-900 A 3.33-30 PHz 14-124 eV 1.0
X-ray: Full 0.25-100 A 30-120 PHz 0.12-50 keV 2.6
[ X-Ray: Soft 2.5-100 A 0.03-1.2 EHz 0.12-5 keV 1.6 ]
[ X-Ray: Hard 0.25-2.5 A 1.2-12 EHz 5-50 keV 1.0 ]
Gamma-ray <0.25 A >12 EHz >50 keV >8.0
[ Gamma-ray: VHE <2.5E-07 A >12 YHz >50 GeV >2.0 ]
Highest-energy srcs 2.5E-10 A 12000 YHz ~50 TeV ---
The metric prefixes are as follows, using Hz as an example:
1 kHz = 103 Hz
1 MHz = 106 Hz
1 GHz = 109 Hz
1 THz = 1012 Hz
1 PHz = 1015 Hz
1 EHz = 1018 Hz
1 ZHz = 1021 Hz
1 YHz = 1024 Hz
Notice that the gamma-ray and radio bands are the broadest, covering many
decades (factors of ten) of the electromagnetic spectrum, while the X-ray band
covers almost three decades, the IR band two decades, and the EUV band about
one decade. The optical band covers only a trivial 0.3 out of the more
than 20 decades of the electromagnetic spectrum!
The HEASARC is the NASA Data
Center with the specific responsibility for archiving EUV,
X-ray and gamma-ray data obtained from observations of cosmic sources
(excluding the Sun) which have been made by satellites operated by NASA
as well as by other space agencies such as ESA, and the German,
Japanese, and UK space agencies.
Web page author and maintainer: Stephen A.
Drake
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Last modified: Monday, 30-Jun-2008 15:29:18 EDT
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