Editing Sunlight (section)

==Composition and power==
[[File:Solar spectrum compared to black-body.gif|thumb|350px|Solar spectrum compared to black-body at 5775&nbsp;K]]

{{See also|Ultraviolet|Infrared|Light}}

The [[frequency spectrum|spectrum]] of the Sun's solar radiation can be compared to [[black-body radiation|that of a black body]]<ref>{{cite journal | doi = 10.1038/156534b0 | volume=156 | title=Departure of Long-Wave Solar Radiation from Black-Body Intensity | year=1945 | journal=Nature | pages=534–535  | last1 = Appleton | first1 = Edward V.| issue=3966 | bibcode=1945Natur.156..534A | s2cid=4092179 }}</ref><ref>Iqbal, M., "An Introduction to Solar Radiation", Academic Press (1983), Chap. 3</ref> with a temperature of about 5,800&nbsp;[[Kelvin|K]]<ref>[http://solarsystem.nasa.gov/planets/profile.cfm?Display=Facts&Object=Sun NASA Solar System Exploration – Sun: Facts & Figures] {{webarchive|url=https://web.archive.org/web/20150703111716/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sun&Display=Facts |date=2015-07-03 }} retrieved 27 April 2011 "Effective Temperature ... 5777&nbsp;K"</ref> (see graph). The Sun emits EM radiation across most of the [[electromagnetic spectrum]]. Although the radiation created in the solar core consists mostly of [[x ray]]s, internal absorption and thermalization convert these super-high-energy [[photon]]s to lower-energy photons before they reach the Sun's surface and are emitted out into space. As a result, the [[photosphere]] of the Sun does not emit much X radiation ([[solar X-rays]]), although it does emit such "hard radiations" as X-rays and even [[gamma rays]] during [[solar flare]]s.<ref>{{cite web|url=http://www.nasa.gov/mission_pages/GLAST/news/highest-energy.html|title=Fermi Detects Solar Flare's Highest-Energy Light|first=Rob|last=Garner|date=24 January 2017|access-date=25 January 2018|url-status=live|archive-url=https://web.archive.org/web/20170517060027/https://www.nasa.gov/mission_pages/GLAST/news/highest-energy.html|archive-date=17 May 2017}}</ref> The quiet (non-flaring) Sun, including its [[Stellar corona|corona]], emits a broad range
of wavelengths: [[X-ray]]s, [[ultraviolet]], [[visible light]], [[infrared]], and [[radio wave]]s.<ref>{{cite web |url= http://www.windows2universe.org/sun/spectrum/multispectral_sun_overview.html |title= The Multispectral Sun, from the National Earth Science Teachers Association |publisher= Windows2universe.org |date= 2007-04-18 |access-date= 2012-02-12 |url-status= live |archive-url= https://web.archive.org/web/20120229041535/http://www.windows2universe.org/sun/spectrum/multispectral_sun_overview.html |archive-date= 2012-02-29 }}</ref> Different depths in the photosphere have different temperatures, and this partially explains the deviations from a black-body spectrum.<ref>See video referenced in the sentence "For more details about the comparison of the black body with the AM0 spectrum, see this video"  at {{cite web |last1=Pietro Altermatt |title=The Extraterrestrial Spectrum |url=https://pvlighthouse.com.au/cms/lectures/altermatt/solar_spectrum/blackbody-radiation |website=PV Lighthouse |publisher=PV Lighthouse Pty. Ltd.}}</ref>

There is also a flux of gamma rays from the quiescent Sun, obeying a [[power law]] between 0.5 and 2.6 [[TeV]]. Some gamma rays are caused by [[cosmic rays]] interacting with the solar atmosphere, but this does not explain these findings.<ref>{{cite journal |last1=Ryan Wilkinson |title=Record-Breaking Detection of Solar Photons |journal=Physics |date=Aug 3, 2023 |volume=16 |doi=10.1103/Physics.16.s107 |bibcode=2023PhyOJ..16.s107W |s2cid=260763644 |url=https://physics.aps.org/articles/v16/s107|doi-access=free }}</ref><ref>{{cite journal |last1=Leah Crane |title=Astronomers have spotted inexplicably bright light coming from the sun |journal=New Scientist |date=Aug 3, 2023 |url=https://www.newscientist.com/article/2386042-astronomers-have-spotted-inexplicably-bright-light-coming-from-the-sun/}}</ref><ref>{{cite journal |last1=A. Albert |title=Discovery of Gamma Rays from the Quiescent Sun with HAWC |journal=Phys. Rev. Lett. |date=Aug 3, 2023 |volume=131 |issue=5 |page=051201 |doi=10.1103/PhysRevLett.131.051201|pmid=37595214 |arxiv=2212.00815 |bibcode=2023PhRvL.131e1201A |s2cid=254221151 }}</ref>

The only direct signature of the nuclear processes in the core of the Sun is via the very weakly interacting [[neutrino]]s.

[[File:Solar spectrum en.svg|thumb|upright=1.25|Solar [[spectral irradiance]] (watts per square metre per nanometre) above atmosphere (yellow) and at surface (red). Extreme UV and X-rays are produced (left of wavelength range) but comprise very small amounts of the Sun's total output power (area under the curve).]]
[[File:Spectral Distribution of Sunlight.svg|thumb|upright=1.25|Spectral distribution of sunlight. The different curves reflect 3 different equally valid ways of characterizing the same sunlight. These curves have peaks at different wavelengths, which demonstrates that the notion of a location where the "peak" amount of sunlight is emitted is not meaningful, and is not a characteristic of the light itself (but is merely an artifact of how the spectrum is represented). Percentiles offer a way of thinking about the distribution of energy which is independent of the representation. 50 percent of solar irradiance is associated with wavelengths less than about 711 nm (based on approximating sunlight by the emissions of a 5775 K blackbody).]]

Although the [[Stellar corona|solar corona]] is a source of [[extreme ultraviolet]] and X-ray radiation, these rays make up only a very small amount of the power output of the Sun (see spectrum at right). The spectrum of nearly all solar [[electromagnetic radiation]] striking the [[Atmosphere of Earth|Earth's atmosphere]] spans a range of 100&nbsp;[[Nanometre|nm]] to about 1&nbsp;[[Millimetre|mm]] (1,000,000&nbsp;nm).{{Citation needed|reason=Specific information with no sourcing|date=March 2017}} This band of significant radiation power can be divided into five regions in increasing order of [[wavelength]]s:<ref>{{cite web
 |last        = Naylor
 |first       = Mark
 |author2     = Kevin C. Farmer
 |title       = Sun damage and prevention
 |work        = Electronic Textbook of Dermatology
 |publisher   = The Internet Dermatology Society
 |date        = 1995
 |url         = http://www.telemedicine.org/sundam/sundam2.4.1.html
 |access-date  = 2008-06-02
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20080705111726/http://telemedicine.org/sundam/sundam2.4.1.html
 |archive-date = 2008-07-05
}}</ref>
* '''Ultraviolet&nbsp;C''' or (UVC) range, which spans a range of 100 to 280&nbsp;nm. The term ''ultraviolet'' refers to the fact that the radiation is at higher frequency than violet light (and, hence, also invisible to the [[human eye]]). Due to absorption by the atmosphere very little reaches Earth's surface<!-- Ocean's surface is not lithosphere. Do not insert junk, please. --->. This spectrum of radiation [[Ultraviolet germicidal irradiation|has germicidal properties]], as used in [[germicidal lamp]]s.
* '''Ultraviolet&nbsp;B''' or (UVB) range spans 280 to 315&nbsp;nm. It is also greatly absorbed by the Earth's atmosphere, and along with UVC causes the [[photochemical reaction]] leading to the production of the [[ozone layer]]. It directly damages DNA and causes [[sunburn]].<ref name="Sunlight and Vitamin D: A global pe">{{cite journal | title = Sunlight and Vitamin D: A global perspective for health. | vauthors = Wacker M, Holick, MF | date = 2013 | journal = Dermato-Endocrinology | volume = 5 | issue = 1 | pages = 51–108 | doi = 10.4161/derm.24494 | pmid = 24494042 | pmc=3897598}}</ref> In addition to this short-term effect it enhances skin ageing and significantly promotes the development of skin cancer,<ref>{{Cite web|title=Radiation: Ultraviolet (UV) radiation
| date = 9 March 2016
| author= World Health Organization 
|url=https://www.who.int/news-room/questions-and-answers/item/radiation-ultraviolet-(uv)|access-date=2023-02-08|language=en}}</ref> but is also required for [[vitamin D]] synthesis in the skin of mammals.<ref name="Sunlight and Vitamin D: A global pe"/>
* '''Ultraviolet&nbsp;A''' or (UVA) spans 315 to 400&nbsp;nm. This band was once{{when|date=December 2016}} held to be less damaging to [[DNA]], and hence is used in cosmetic artificial [[sun tanning]] ([[tanning booth]]s and [[tanning bed]]s) and [[PUVA]] therapy for [[psoriasis]]. However, UVA is now known to cause significant damage to DNA via indirect routes (formation of [[free radicals]] and [[reactive oxygen species]]), and can cause cancer.<ref>{{cite journal|title=Ultraviolet Radiation Exposure and Its Impact on Skin Cancer Risk|date=1 August 2017|pmc = 5036351|last1 = Watson|first1 = M.|last2 = Holman|first2 = D. M.|last3 = Maguire-Eisen|first3 = M.|journal = Seminars in Oncology Nursing|volume = 32|issue = 3|pages = 241–254|doi = 10.1016/j.soncn.2016.05.005|pmid = 27539279}}</ref>
* '''[[Visible light|Visible]] range''' or '''light''' spans 380 to 700&nbsp;nm.<ref>{{Cite web | url=https://science.nasa.gov/ems/09_visiblelight |title = Visible Light {{pipe}} Science Mission Directorate| date=10 August 2016 }}</ref> As the name suggests, this range is visible to the naked eye. 
* '''[[Infrared]]''' range that spans 700&nbsp;nm to 1,000,000&nbsp;nm (1&nbsp;[[Millimeter|mm]]). It comprises an important part of the electromagnetic radiation that reaches Earth. Scientists divide the infrared range into three types on the basis of wavelength:
** Infrared-A: 700&nbsp;nm to 1,400&nbsp;nm
** Infrared-B: 1,400&nbsp;nm to 3,000&nbsp;nm
** Infrared-C: 3,000&nbsp;nm to 1&nbsp;mm.

The sunlight reaching Earth's surface is 49.4% infrared, 42.3% visible, and 8% ultraviolet.<ref name="fondriest">{{cite web |title=Solar Radiation and Photosynethically Active Radiation |url=https://www.fondriest.com/environmental-measurements/parameters/weather/solar-radiation/ |publisher=Fondriest Environmental |access-date=7 March 2025 |date=March 21, 2014}}</ref>

It is sometimes asserted that the Sun's maximum output is in the visible range. However, this statement is a misconception based on only seeing the solar spectral irradiance plotted on a per-wavelength basis. When plotted that way, the power spectral density of sunlight peaks at a wavelength of about 501 nm, which is in the visible range. However, the solar spectral irradiance can with equal validity be calculated on a per-frequency basis, in which case the maximum is at {{val|3.40e14|u=Hz}}, corresponding to a wavelength of about 882 nm, which is in the near infrared (Infrared-A) range. Counterintuitively, it is not meaningful to assert that the solar output is greatest at some precise location in the spectrum.<ref name="oomisconcept">{{cite web |last1=Mobley |first1=Curtis |title=A Common Misconception |url=https://www.oceanopticsbook.info/view/light-and-radiometry/level-2/common-misconception |website=Ocean Optics |access-date=6 March 2025}}</ref>

===Published tables===
Tables of direct solar radiation on various slopes from 0 to 60 degrees north latitude, in calories per square centimetre, issued in 1972 and published by Pacific Northwest Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture, Portland, Oregon, USA, appear on the web.<ref>
{{cite web
 |url         = https://www.fs.usda.gov/pnw/pubs/pnw_rp142.pdf
 |title       = Direct Solar Radiation On Various Slopes From 0 To 60 Degrees North Latitude
 |author      = John Buffo
 |author2     = Leo J. Fritschen
 |author3     = James L. Murphy
 |publisher   = Pacific Northwest Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture, Portland, Oregon, USA
 |date        = 1972
 |access-date  = 15 Jan 2014
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20131127124158/http://www.fs.fed.us/pnw/pubs/pnw_rp142.pdf
 |archive-date = 2013-11-27
}}
</ref>