Changes

Light in a vacuum propagates at a maximum finite speed, defined above, and denoted by the symbol ''c''. While passing through any other transparent medium, the speed of light slows to some fraction of ''c''.  The reduction of the speed of light traveling in a transparent medium is indicated by the [[refractive index]], ''n'', which is defined as:

Light in a vacuum propagates at a maximum finite speed, defined above, and denoted by the symbol ''c''. While passing through any other transparent medium, the speed of light slows to some fraction of ''c''.  The reduction of the speed of light traveling in a transparent medium is indicated by the [[refractive index]], ''n'', which is defined as:

:<math> n = \frac{c}{v} \;\!</math>

<math> n = \frac{c}{v} \;\!</math>

where ''v'' denotes the speed that light travels in the transparent medium.

where ''v'' denotes the speed that light travels in the transparent medium.

== Light pressure ==

== Light pressure ==

Light pushes on objects in its way, just as the wind would do. This pressure is most easily explainable in particle theory: photons hit and transfer their momentum. Light pressure can cause [[asteroid]]s to spin faster, [http://discovermagazine.com/2004/feb/asteroids-get-spun-by-the-sun/] Asteroids Get Spun By the Sun) acting on their irregular shapes as on the vanes of a [[windmill]]. The possibility to make [[solar sail]]s that would accelerate spaceships in space is also under investigation.{{Fact|date=May 2007}}

Light pushes on objects in its way, just as the wind would do. This pressure is most easily explainable in particle theory: photons hit and transfer their momentum. Light pressure can cause [[asteroid]]s to spin faster, [http://discovermagazine.com/2004/feb/asteroids-get-spun-by-the-sun/] Asteroids Get Spun By the Sun) acting on their irregular shapes as on the vanes of a [[windmill]]. The possibility to make [[solar sail]]s that would accelerate spaceships in space is also under investigation.

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>P. Lebedev, Untersuchungen über die Druckkräfte des Lichtes, Ann. Phys. 6, 433 (1901).</ref> This should not be confused with the [[Nichols radiometer]], in which the motion ''is'' directly caused by light pressure.<ref>Nichols, E.F & Hull, G.F. (1903) [http://books.google.com/books?id=8n8OAAAAIAAJ&pg=RA5-PA327&dq=torsion+balance+radiation The Pressure due to Radiation], ''The Astrophysical Journal'',Vol.17 No.5, p.315-351</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>P. Lebedev, Untersuchungen über die Druckkräfte des Lichtes, Ann. Phys. 6, 433 (1901).</ref> This should not be confused with the [[Nichols radiometer]], in which the motion ''is'' directly caused by light pressure.<ref>Nichols, E.F & Hull, G.F. (1903) [http://books.google.com/books?id=8n8OAAAAIAAJ&pg=RA5-PA327&dq=torsion+balance+radiation The Pressure due to Radiation], ''The Astrophysical Journal'',Vol.17 No.5, p.315-351</ref>