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Light pressure {{#invoke:main|main}} 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 c, the speed of light.{{#invoke:String|rep| |1}} Due to the magnitude of c, the effect of light pressure is negligible for everyday objects.{{#invoke:String|rep| |1}} For example, a one-milliwatt laser pointer exerts a force of about 3.3 piconewtons on the object being illuminated; thus, one could lift a U.S. penny with laser pointers, but doing so would require about 30 billion 1-mW laser pointers.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>{{#invoke:String|rep| |1}} 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, nano-opto-mechanical systems research at Yale University.</ref>

At larger scales, light pressure can cause asteroids to spin faster,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> acting on their irregular shapes as on the vanes of a windmill.{{#invoke:String|rep| |1}} The possibility of making solar sails that would accelerate spaceships in space is also under investigation.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</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 (slight) motion caused by torque (though not enough for full rotation against friction) is directly caused by light pressure.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>


Light sections
Intro  Electromagnetic spectrum and visible light   Speed of light   Optics  Units and measures  Light pressure  Historical theories about light, in chronological order  See also  Notes  References  External links  

Light pressure
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