Lysets hastighed: Forskelle mellem versioner
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[[Fil:Speed of light from Earth to Moon 400px.gif|thumb|Lysets færd fra Jorden til Månen i realtid]]
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'''Lysets hastighed''' eller '''lysets fart''' er den [[fart]], hvormed elektromagnetiske svingninger udbreder sig i et medium. I [[vakuum]] er [[lys]]ets hastighed maksimal, og siden 1983 er [[meter]]en defineret som den [[længde]], lyset tilbagelægger i vakuum på 1/299.792.458 [[sekund]]er<ref>[http://www.bipm.org/en/CGPM/db/17/1/ www.bipm.org/en/CGPM/db/17/1/]</ref>. Det vil sige, at lysets hastighed er 299.792.458 meter per sekund eller tæt på 300.000 km/s. I andre medier end vakuum er lysets hastighed mindre, se [[brydningsindeks]]. I [[partikel (fysik)|partikelmodellen]] opfattes lys som en strøm af masseløse partikler, såkaldte [[foton]]er, og lysets hastighed bliver så den hastighed, hvormed fotoner bevæger sig. Fotoner er masseløse men kan overføre impuls (bevægelsesmængde).
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