Editing Light (section)

==Light pressure==
{{Main|Radiation pressure}}
Light exerts physical pressure on objects in its path, a phenomenon which can be deduced by [[Maxwell's equations]], but can be more easily explained by the particle nature of light: photons strike and transfer their momentum. Light pressure is equal to the power of the light beam divided by ''[[speed of light|c]]'', the speed of light.{{Spaces}} Due to the magnitude of ''c'', the effect of light pressure is negligible for everyday objects.{{Spaces}} For example, a one-milliwatt [[laser pointer]] exerts a force of about 3.3 [[newton (unit)|piconewtons]] on the object being illuminated; thus, one could lift a [[penny (United States coin)|U.S. penny]] with laser pointers, but doing so would require about 30&nbsp;billion 1-mW laser pointers.<ref>{{cite journal |last=Tang |first=Hong |title=May The Force of Light Be With You |journal=IEEE Spectrum |date=1 October 2009 |volume=46 |issue=10 |pages=46–51 |doi=10.1109/MSPEC.2009.5268000|s2cid=7928030 }}</ref>{{Spaces}} However, in [[nanometre]]-scale applications such as [[nanoelectromechanical systems]] (NEMS), the effect of light pressure is more significant and exploiting light pressure to drive NEMS mechanisms and to flip nanometre-scale physical switches in integrated circuits is an active area of research.<ref>See, for example, [http://www.eng.yale.edu/tanglab/research.htm nano-opto-mechanical systems research at Yale University] {{Webarchive|url=https://web.archive.org/web/20100625042036/http://www.eng.yale.edu/tanglab/research.htm |date=25 June 2010 }}.</ref> At larger scales, light pressure can cause [[asteroid]]s to spin faster,<ref>{{cite web |url=http://discovermagazine.com/2004/feb/asteroids-get-spun-by-the-sun/ |title=Asteroids Get Spun By the Sun |first=Kathy A. |last=Svitil |website=Discover Magazine |date=5 February 2004 |access-date=8 May 2007 |archive-date=9 October 2012 |archive-url=https://web.archive.org/web/20121009045611/http://discovermagazine.com/2004/feb/asteroids-get-spun-by-the-sun/ |url-status=live }}</ref> acting on their irregular shapes as on the vanes of a [[windmill]].{{Spaces}} The possibility of making [[solar sail]]s that would accelerate spaceships in space is also under investigation.<ref>{{cite web |url=http://www.nasa.gov/vision/universe/roboticexplorers/solar_sails.html |title=Solar Sails Could Send Spacecraft 'Sailing' Through Space |website=NASA |date=31 August 2004 |access-date=30 May 2008 |archive-date=21 October 2012 |archive-url=https://web.archive.org/web/20121021035846/http://www.nasa.gov/vision/universe/roboticexplorers/solar_sails.html |url-status=live }}</ref><ref>{{cite web |url=http://www.nasa.gov/centers/marshall/news/news/releases/2004/04-208.html |title=NASA team successfully deploys two solar sail systems |website=NASA |date=9 August 2004 |access-date=30 May 2008 |archive-date=14 June 2012 |archive-url=https://web.archive.org/web/20120614013757/http://www.nasa.gov/centers/marshall/news/news/releases/2004/04-208.html |url-status=live }}</ref>

Although the motion of the [[Crookes radiometer]] was originally attributed to light pressure, this interpretation is incorrect; the characteristic Crookes rotation is the result of a partial vacuum.<ref>{{cite journal |author-link=Pyotr Lebedev |first=P. |last=Lebedew |title=Untersuchungen über die Druckkräfte des Lichtes |journal=Annalen der Physik |volume=6 |issue=11 |pages=433–458 |year=1901 |doi=10.1002/andp.19013111102 |bibcode=1901AnP...311..433L |url=https://zenodo.org/record/1424005 |access-date=29 July 2022 |archive-date=6 June 2022 |archive-url=https://web.archive.org/web/20220606184159/https://zenodo.org/record/1424005 |url-status=live }}</ref> This should not be confused with the [[Nichols radiometer]], in which the (slight) motion caused by torque (though not enough for full rotation against friction) ''is'' directly caused by light pressure.<ref>{{cite journal |last1=Nichols |first1=E.F |last2=Hull |first2=G.F. |year=1903 |url=https://books.google.com/books?id=8n8OAAAAIAAJ&q=torsion+balance+radiation&pg=RA5-PA327 |title=The Pressure due to Radiation |journal=The Astrophysical Journal |volume=17 |pages=315–351 |issue=5 |bibcode=1903ApJ....17..315N |doi=10.1086/141035 |access-date=15 November 2020 |archive-date=8 October 2022 |archive-url=https://web.archive.org/web/20221008031820/https://books.google.com/books?id=8n8OAAAAIAAJ&q=torsion+balance+radiation&pg=RA5-PA327 |url-status=live |doi-access=free }}</ref>
As a consequence of light pressure, [[Albert Einstein|Einstein]] in 1909 predicted the existence of "radiation friction" which would oppose the movement of matter.<ref>{{cite book |last=Einstein, A. |chapter=Über die Entwicklung unserer Anschauungen über das Wesen und die Konstitution der Strahlung |trans-chapter=On the development of our views concerning the nature and constitution of radiation |title=The Collected Papers of Albert Einstein |volume=2 |year=1989 |orig-date=1909 |publisher=Princeton University Press |location=Princeton, New Jersey |page=391}}</ref> He wrote, "radiation will exert pressure on both sides of the plate. The forces of pressure exerted on the two sides are equal if the plate is at rest. However, if it is in motion, more radiation will be reflected on the surface that is ahead during the motion (front surface) than on the back surface. The backwardacting force of pressure exerted on the front surface is thus larger than the force of pressure acting on the back. Hence, as the resultant of the two forces, there remains a force that counteracts the motion of the plate and that increases with the velocity of the plate. We will call this resultant 'radiation friction' in brief."

Usually light momentum is aligned with its direction of motion. However, for example in [[evanescent wave]]s momentum is transverse to direction of propagation.<ref>{{Cite journal|last1=Antognozzi|first1=M.|last2=Bermingham|first2=C. R.|last3=Harniman|first3=R. L.|last4=Simpson|first4=S.|last5=Senior|first5=J.|last6=Hayward|first6=R.|last7=Hoerber|first7=H.|last8=Dennis|first8=M. R.|last9=Bekshaev|first9=A. Y.|date=August 2016|title=Direct measurements of the extraordinary optical momentum and transverse spin-dependent force using a nano-cantilever|journal=Nature Physics|volume=12|issue=8|pages=731–735|doi=10.1038/nphys3732|issn=1745-2473|arxiv=1506.04248|bibcode=2016NatPh..12..731A|s2cid=52226942}}</ref>