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The speed of light is slower in any medium which is not a vacuum, and varies from medium to medium. This variation gives rise to (as a result of [[quantum mechanics]], particularly the concept of a path of least action) the phenomenon of [[refraction]]. When a charged particle exceeds the speed of light in the medium in which it is travelling, it emits [[Cherenkov Radiation]].

The speed of light is slower in any medium which is not a vacuum, and varies from medium to medium. This variation gives rise to (as a result of [[quantum mechanics]], particularly the concept of a path of least action) the phenomenon of [[refraction]]. When a charged particle exceeds the speed of light in the medium in which it is travelling, it emits [[Cherenkov Radiation]].

Since the speed of light in a vacuum is constant, it can be used to define distances as well. The distance that light travels in one year is known as a [[light-year]], which is about 6 million million (6x10¹²) miles.  For instance, the nearest large galaxy to our own [[Milky Way Galaxy]] is the [[Andromeda Galaxy]], which astronomers have measured to be about 2.5 million light years away.

Since the speed of light in a vacuum is constant, it can be used to define distances as well. The distance that light travels in one year is known as a [[light-year]], which is about 6 million million (6x10¹²) miles.  For instance, the nearest large galaxy to our own [[Milky Way Galaxy]] is the [[Andromeda Galaxy]], which astronomers have measured to be about 2.5 million light years away, meaning visible light from the galaxy has taken 2.5 million years to reach Earth.

The speed of light raises several questions for [[Young Earth Creationism|Young Earth Creationists]] including that of the [[starlight problem]].  An Australian young Earth creationist named Barry Setterfield proposed the mechanism of [[c-decay]] <ref>http://www.setterfield.org/vacuum.html</ref> to allow light from sources more distant than 6-10,000 light years to have made the transit to Earth.

The speed of light raises several questions for [[Young Earth Creationism|Young Earth Creationists]] including that of the [[starlight problem]].  An Australian young Earth creationist named Barry Setterfield proposed the mechanism of [[c-decay]] <ref>http://www.setterfield.org/vacuum.html</ref> to allow light from sources more distant than 6-10,000 light years to have made the transit to Earth. No scientists believe this unproven claim.<ref>http://www.talkorigins.org/faqs/c-decay.html</ref>

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