{{Short description|Physical process by which matter takes up a photon's energy and stores it}} {{More footnotes|date=November 2010}}
[[File:Spectroscopy overview.svg|thumb|upright=2|right|An overview of absorption of [[electromagnetic radiation]]. This example shows the general principle using [[visible spectrum|visible light]] as a specific example. A white [[light source]]—emitting light of multiple [[wavelength]]s—is focused on a sample (the pairs of [[complementary color]]s are indicated by the yellow dotted lines). Upon striking the sample, [[photon]]s that match the [[energy gap]] of the [[molecule]]s present (green light in this example) are ''absorbed'', exciting the molecules. Other photons are scattered (not shown here) or transmitted unaffected; if the radiation is in the visible region (400–700 nm), the transmitted light appears as the complementary color (here red). By recording the [[attenuation]] of light for various wavelengths, an [[absorption spectroscopy|absorption spectrum]] can be obtained.]]
In [[physics]], '''absorption''' of [[electromagnetic radiation]] is how [[matter]] (typically [[electrons]] bound in [[atom]]s) takes up a [[photon]]'s [[energy]]—and so transforms [[radiant energy|electromagnetic energy]] into [[internal energy]] of the absorber (for example, [[thermal energy]]).<ref>{{Cite journal|title=Absorption of electromagnetic radiation|url=http://accessscience.com/content/Absorption-of-electromagnetic-radiation/001600|date=September 2019|first=Christopher S.|last=Baird|journal=AccessScience|publisher=McGraw-Hill|access-date=17 June 2023|doi=10.1036/1097-8542.001600|url-access=subscription}}</ref>
A notable effect of the absorption of electromagnetic radiation is [[attenuation]] of the radiation; attenuation is the gradual reduction of the [[Intensity (physics)|intensity]] of [[light waves]] as they [[wave propagation|propagate]] through a medium.
Although the absorption of waves does not usually depend on their intensity (linear absorption), in certain conditions ([[optics]]) the medium's transparency changes by a factor that varies as a function of wave intensity, and [[saturable absorption]] (or nonlinear absorption) occurs.
==Quantifying absorption== {{Main|Mathematical descriptions of opacity}}
Many approaches can potentially quantify radiation absorption, with key examples following.
* The absorption coefficient along with some closely related derived quantities * The [[attenuation coefficient]] (NB used infrequently with meaning synonymous with "absorption coefficient"){{citation needed|date=October 2018}} * The [[Molar attenuation coefficient]] (also called "molar absorptivity"), which is the absorption coefficient divided by molarity (see also [[Beer–Lambert law]]) * The [[mass attenuation coefficient]] (also called "mass extinction coefficient"), which is the absorption coefficient divided by density * The [[absorption cross section]] and [[scattering cross-section]], related closely to the absorption and attenuation coefficients, respectively * [[Extinction (astronomy)|"Extinction" in astronomy]], which is equivalent to the attenuation coefficient * Other measures of radiation absorption, including [[penetration depth]] and [[skin effect]], [[propagation constant]], [[propagation constant#Attenuation constant|attenuation constant]], [[propagation constant#Phase constant|phase constant]], and complex [[wavenumber]], [[Refractive index|complex refractive index]] and [[refractive index#Dispersion and absorption|extinction coefficient]], [[Relative permittivity|complex dielectric constant]], [[electrical resistivity and conductivity]]. * Related measures, including [[absorbance]] (also called "optical density") and [[optical depth]] (also called "optical thickness")
All these quantities measure, at least to some extent, how well a medium absorbs radiation. Which among them practitioners use varies by field and technique, often due simply to the convention.
==Measuring absorption== The [[absorbance]] of an object quantifies how much of the incident light is absorbed by it (instead of being [[reflection (physics)|reflected]] or [[refraction|refracted]]). This may be related to other properties of the object through the [[Beer–Lambert law]].
Precise measurements of the absorbance at many wavelengths allow the identification of a substance via [[absorption spectroscopy]], where a sample is illuminated from one side, and the intensity of the light that exits from the sample in every direction is measured. A few examples of absorption are [[ultraviolet–visible spectroscopy]], [[infrared spectroscopy]], and [[X-ray absorption spectroscopy]].
==Applications== [[File:Openstax Astronomy EM spectrum and atmosphere.jpg|thumb|right|Rough plot of Earth's atmospheric [[transmittance]] (or opacity) to various wavelengths of electromagnetic radiation, including [[visible light]]]]
Understanding and measuring the absorption of electromagnetic radiation has a variety of applications.
* In [[radio propagation]], it is represented in [[non-line-of-sight propagation]]. For example, see [[computation of radiowave attenuation in the atmosphere|computation of radio wave attenuation in the atmosphere]] used in satellite link design. * In [[meteorology]] and [[climatology]], global and local temperatures depend in part on the absorption of radiation by [[atmospheric gases]] (such as in the [[greenhouse effect]]) and land and ocean surfaces (see [[albedo]]). * In [[medicine]], [[X-ray]]s are absorbed to different extents by different tissues ([[bone]] in particular), which is the basis for [[Projectional radiography|X-ray imaging]]. * In [[chemistry]] and [[materials science]], different materials and molecules absorb radiation to different extents at different frequencies, which allows for material identification. * In [[optics]], sunglasses, colored filters, dyes, and other such materials are designed specifically with respect to which visible wavelengths they absorb, and in what proportions they are in. * In [[biology]], photosynthetic organisms require that light of the appropriate wavelengths be absorbed within the active area of [[chloroplast]]s, so that the [[light]] energy can be converted into [[chemical energy]] within sugars and other molecules. * In [[physics]], the D-region of Earth's [[ionosphere]] is known to significantly absorb radio signals that fall within the high-frequency electromagnetic spectrum. * In nuclear physics, absorption of nuclear radiations can be used for measuring the fluid levels, densitometry or thickness measurements.<ref>{{cite journal|last1=M. Falahati |display-authors=et al|title=Design, modelling and construction of a continuous nuclear gauge for measuring the fluid levels|journal=Journal of Instrumentation|date=2018|volume=13|issue=2|page=02028|doi=10.1088/1748-0221/13/02/P02028|bibcode=2018JInst..13P2028F|s2cid=125779702 }}</ref>
In scientific literature is known a system of mirrors and lenses that with a laser "can enable any material to absorb all light from a wide range of angles."<ref>{{cite web|url=https://physicsworld.com/a/anti-laser-enables-near-perfect-light-absorption/|title=Anti-laser enables near-perfect light absorption|date=August 31, 2022|publisher=[[Physics World]]}}</ref>
==See also== *[[Absorption spectroscopy]] *[[Albedo]] *[[Attenuation]] *[[Electromagnetic absorption by water]] *[[Hydroxyl ion absorption]] *[[Optoelectronics]] *[[Photoelectric effect]] *[[Photosynthesis]] *[[Physical crystallography before X-rays#Absorption and pleochroism|Physical crystallography before X-rays]] *[[Solar cell]] *[[Spectral line]] *[[Total absorption spectroscopy]] *[[Ultraviolet-visible spectroscopy]]
==References== {{Reflist}}
*{{cite book | last= Thomas | first= Michael E. | title= Optical Propagation in Linear Media: Atmospheric Gases and Particles, Solid-State Components, and Water | journal= Optical Propagation in Linear Media: Atmospheric Gases and Particles | publisher= Oxford University Press, USA | date= January 2006 | pages= 3... (Chapter 1, 2, 7) | url= https://books.google.com/books?id=EEodkQqPGs4C&q=absorption | isbn= 978-0-19-509161-8| bibcode= 2006oplm.book.....T }} *{{cite web | author1= ProfHoff, Ken Mellendorf | author2= Vince Calder | title= Reflection and Absorption | work= Physics Archive - Ask a scientist | publisher= [[Argonne National Laboratory]] | date= November 2010 | url= http://www.newton.dep.anl.gov/askasci/phy00/phy00232.htm | access-date= 2010-11-14 | archive-date= 2010-11-21 | archive-url= https://web.archive.org/web/20101121094449/http://www.newton.dep.anl.gov/askasci/phy00/phy00232.htm | url-status= dead }}
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[[Category:Scattering, absorption and radiative transfer (optics)]] [[Category:Electromagnetic radiation]] [[Category:Glass physics]] [[Category:Radiation]] [[Category:Spectroscopy]]