# Electronvolt

{{#invoke:Hatnote|hatnote}} In physics, the electron volt (symbol eV; also written electronvolt[1][2]) is a unit of energy equal to approximately Template:Val joules (symbol J). By definition, it is the amount of energy gained (or lost) by the charge of a single electron moved across an electric potential difference of one volt. Thus it is 1 volt (1 joule per coulomb, Template:Val) multiplied by the elementary charge (e, or Template:Val). Therefore, one electron volt is equal to Template:Val.[3] Historically, the electron volt was devised as a standard unit of measure through its usefulness in electrostatic particle accelerator sciences because a particle with charge q has an energy E = qV after passing through the potential V; if q is quoted in integer units of the elementary charge and the terminal bias in volts, one gets an energy in eV.

Photon frequency vs. energy per particle in electronvolts. The energy of a photon varies only with the frequency of the photon, related by speed of light constant. This contrasts with a massive particle of which the energy depends on its velocity and rest mass.[4][5][6]

Legend
 γ: Gamma rays MIR: Mid infrared HF: High freq. HX: Hard X-rays FIR: Far infrared MF: Medium freq. SX: Soft X-rays Radio waves LF: Low freq. EUV: Extreme ultraviolet EHF= Extremely high freq. VLF: Very low freq. NUV: Near ultraviolet SHF= Super high freq. VF/ULF: Voice freq. Visible light UHF= Ultra high freq. SLF: Super low freq. NIR: Near Infrared VHF= Very high freq. ELF: Extremely low freq. Freq: Frequency

## Properties

Energy of photons in the visible spectrum

The energy E, frequency v, and wavelength λ of a photon are related by

${\displaystyle E=h\nu ={\frac {hc}{\lambda }}={\frac {(4.13566733\times 10^{-15}\,{\mbox{eV}}\,{\mbox{s}})(299\,792\,458\,{\mbox{m/s}})}{\lambda }}}$

where h is the Planck constant, c is the speed of light. This reduces to

${\displaystyle E{\mbox{(eV)}}={\frac {1239.84187\,{\mbox{eV}}\,{\mbox{nm}}}{\lambda \ {\mbox{(nm)}}}}.}$

A photon with a wavelength of Template:Val (green light) would have an energy of approximately Template:Val. Similarly, Template:Val would correspond to an infrared photon of wavelength Template:Val, and so on.

## Scattering experiments

In a low-energy nuclear scattering experiment, it is conventional to refer to the nuclear recoil energy in units of eVr, keVr, etc. This distinguishes the nuclear recoil energy from the "electron equivalent" recoil energy (eVee, keVee, etc.) measured by scintillation light. For example, the yield of a phototube is measured in phe/keVee (photoelectrons per keV electron-equivalent energy). The relationship between eV, eVr, and eVee depends on the medium the scattering takes place in, and must be established empirically for each material.

## Notes and references

1. IUPAC Gold Book, p. 75
2. SI brochure, Sec. 4.1 Table 7
3. Template:Cite web
4. What is Light?UC Davis lecture slides
5. Template:Cite web
6. Template:Cite web
7. Template:Cite web
8. Barrow, J. D. "Natural Units Before Planck." Quarterly Journal of the Royal Astronomical Society 24 (1983): 24.
9. Template:Cite web
10. Template:Cite web
11. Open Questions in Physics. German Electron-Synchrotron. A Research Centre of the Helmholtz Association. Updated March 2006 by JCB. Original by John Baez.
12. Ice-bound hunter sees first hint of cosmic neutrinos New Scientist. 22:49 24 April 2013. by Anil Ananthaswamy.
13. Glossary - CMS Collaboration, CERN
14. Template:Cite web