### What are the Energy Range Definitions for the Various Types of Electromagnetic Radiation?

Last Update: March 18th, 2021

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 μm = 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 ν (in Hz) is E = h x ν, where h, Planck's Constant, is 6.6261 x 10-27 in cgs units. The fundamental relation between wavelength λ (in cm) and frequency f (in Hz) is c = λ 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       ---           ---
of the Earth
Radio: meter+      > 100 cm       <300 MHz       ---          >3.0
Radio: cm + dm     1-100 cm     0.3-30 GHZ       ---           3.0
Radio: mm          0.1-1 cm     30-300 GHz       ---           2.0
Sub-mm/THz      0.01-0.1 cm      0.3-3 THz   0.001-0.01 eV     2.0
IR              1-100 um      3-300 THz     0.01-1.2 eV     3.0
Near-IR     7000-10000 A   0.3-0.43 PHz      1.2-1.8 eV    1
Visible      3200-7000 A  0.43-0.94 PHz      1.8-3.9 eV     1
UV            900-3200 A  0.94-3.33 PHz       3.9-14 eV     1
```
```     EUV            100-900 A    3.33-30 PHz       14-124 eV     1.5
X-ray: Full   0.25-100 A     30-120 PHz     0.12-50 keV     3

[  X-Ray: Soft    2.5-100 A   0.03-1.2 EHz      0.12-5 keV     2.5  ]
[  X-Ray: Hard   0.25-2.5 A     1.2-12 EHz        5-50 keV     2.0  ]

Gamma-ray        <0.25 A        >12 EHz       >50 keV      >8.5

[  Gamma-ray: VHE   <2.5E-07 A     >12 YHz       >50 GeV      >2.5  ]

Highest-energy*   3.1E-11 A       9.7E4 YHz       ~400 TeV*       <1
cosmic photon
```

* 400 TeV = 640 erg The metric prefixes are as follows, using Hz as an example:

```Prefix      Example of unit

kilo        1 kHz = 103 Hz
Mega        1 MHz = 106 Hz
Giga        1 GHz = 109 Hz
Tera        1 THz = 1012 Hz
Peta        1 PHz = 1015 Hz
Exa         1 EHz = 1018 Hz
Zetta       1 ZHz = 1021 Hz
Yotta       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 decades 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. The HEASARC is also responsible for cosmic microwave background data sets, such as those from the COBE and WMAP missions, which typically are from observations made in the sub-mm and mm bands.

* Note that there are particles (so-called cosmic rays) which can have much greater energies than the highest energy cosmic photons ever detected: cosmic rays with energies of ~ 1020 eV (100 EeV or 1.6 x 108 erg or 16 J) have been detected. There is a physical process that restricts particles with energies above E ≳ 5 x 1019 eV as they propagate through the Universe, which is called the Greisen-Zatsepin-Kuzmin (GZK) limit, that is caused by their interaction with the cosmic microwave background via the pion-production process.

Web page author and maintainer: Antara R. Basu-Zych

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