Editing Light (section)

==Light sources== <!-- This section is linked from [[Source]] -->
{{Redirect|Lightsource|the solar energy developer named Lightsource|Lightsource Renewable Energy|a particle accelerator used to generate X-rays|Synchrotron light source}}
{{Further|List of light sources}}

There are many sources of light. A body at a given temperature emits a characteristic spectrum of [[black-body radiation]]. A simple thermal source is [[sunlight]], the radiation emitted by the [[chromosphere]] of the [[Sun]] at around {{cvt|6000|K|C F|lk=on}}. Solar radiation peaks in the visible region of the [[electromagnetic spectrum]] when plotted in wavelength units,<ref>{{cite web |url=http://thulescientific.com/LYNCH%20%26%20Soffer%20OPN%201999.pdf |title=Spectrum and the Color Sensitivity of the Eye |website=Thulescientific.com |access-date=29 August 2017 |archive-date=5 July 2010 |archive-url=https://web.archive.org/web/20100705070506/http://thulescientific.com/LYNCH%20%26%20Soffer%20OPN%201999.pdf |url-status=live}}</ref> and roughly 44% of the radiation that reaches the ground is visible.<ref>{{cite web |url=http://rredc.nrel.gov/solar/spectra/am1.5/ |title=Reference Solar Spectral Irradiance: Air Mass 1.5 |access-date=12 November 2009 |archive-date=12 May 2019 |archive-url=https://web.archive.org/web/20190512190812/https://rredc.nrel.gov/solar//spectra/am1.5/ |url-status=live}}</ref> Another example is [[incandescent light bulb]]s, which emit only around 10% of their energy as visible light and the remainder as infrared. A common thermal light source in history is the glowing solid particles in [[fire|flames]], but these also emit most of their radiation in the infrared and only a fraction in the visible spectrum.

The peak of the black-body spectrum is in the deep infrared, at about 10 [[micrometre]] wavelength, for relatively cool objects like human beings. As the temperature increases, the peak shifts to shorter wavelengths, producing first a red glow, then a white one and finally a blue-white colour as the peak moves out of the visible part of the spectrum and into the ultraviolet. These colours can be seen when metal is heated to "red hot" or "white hot". Blue-white [[thermal emission]] is not often seen, except in stars (the commonly seen pure-blue colour in a [[natural gas|gas]] flame or a [[welder]]'s torch is in fact due to molecular emission, notably by CH radicals emitting a wavelength band around 425&nbsp;nm and is not seen in stars or pure thermal radiation).

Atoms emit and absorb light at characteristic energies. This produces "[[emission line]]s" in the spectrum of each atom. [[Emission (electromagnetic radiation)|Emission]] can be [[spontaneous emission|spontaneous]], as in [[light-emitting diode]]s, [[gas discharge]] lamps (such as [[neon lamp]]s and [[neon sign]]s, [[mercury-vapor lamp]]s, etc.) and flames (light from the hot gas itself—so, for example, [[sodium]] in a gas flame emits characteristic yellow light). Emission can also be [[stimulated emission|stimulated]], as in a [[laser]] or a microwave [[maser]].

Deceleration of a free charged particle, such as an [[electron]], can produce visible radiation: [[cyclotron radiation]], [[synchrotron radiation]] and [[bremsstrahlung]] radiation are all examples of this. Particles moving through a medium faster than the speed of light in that medium can produce visible [[Cherenkov radiation]]. Certain chemicals produce visible radiation by [[chemoluminescence]]. In living things, this process is called [[bioluminescence]]. For example, [[firefly|fireflies]] produce light by this means and boats moving through water can disturb plankton which produce a glowing wake.

Certain substances produce light when they are illuminated by more energetic radiation, a process known as [[fluorescence]]. Some substances emit light slowly after excitation by more energetic radiation. This is known as [[phosphorescence]]. Phosphorescent materials can also be excited by bombarding them with subatomic particles. [[Cathodoluminescence]] is one example. This mechanism is used in [[cathode-ray tube]] [[television set]]s and [[computer monitor]]s.

[[File:Colorful artificial lighting at night.jpg|thumb|250px|[[Hong Kong]] illuminated by colourful artificial [[lighting]]]]

Certain other mechanisms can produce light:
* [[Electroluminescence]]
* [[Scintillation (physics)|Scintillation]]
* [[Sonoluminescence]]
* [[Triboluminescence]]

When the concept of light is intended to include very-high-energy photons (gamma rays), additional generation mechanisms include:
* Particle–[[antiparticle]] annihilation
* [[Radioactive decay]]