Scientist predict that light moves 3x108 m/s in a vacuum. Then, when light hits a medium like air, it goes into a medium with a different index and travels slower (I believe it indexes at 1.007 in air). So my question is, how have scientists come to the conclusion that light travels as fast as it does in a vacuum, because can't we only observe it through a medium?

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    $\begingroup$ @zibadawatimmy But it was discovered by an astronomer thanks to observations of Jupiter's moons. 220,000 km/s he measured, not bad back in 1676! Space is basically an extreme physics lab and physics as such was discovered only thanks to astronomical observations. So I don't think question is off topic. $\endgroup$
    – LocalFluff
    Commented Jun 20, 2015 at 9:03
  • $\begingroup$ @LocalFluff That's kind of playing fast and loose with the meaning. You can also measure the speed of light with a chocolate bar and a microwave, but that doesn't make it an appropriate question on the Cooking SE. If the question was specifically about the history of the measurement, and how it has been measured astrophysically, then sure, appropriate question. But asked this way? I don't think so. $\endgroup$
    – zibadawa timmy
    Commented Jun 20, 2015 at 9:50
  • $\begingroup$ @zibadawatimmy But the speed was the first time "come up with" thanks to an astronomical method. I just don't find it OT at all to ask an astrophysicist about how the speed of light is measured. $\endgroup$
    – LocalFluff
    Commented Jun 20, 2015 at 10:45
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    $\begingroup$ I'm voting to close as unclear. can't we only observe it through a medium? I don't understand what is being asked here. Nothing prevents us from observing light in a vacuum. $\endgroup$
    – user466
    Commented Jun 21, 2015 at 18:34
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    $\begingroup$ I don't think the OP is asking about the history behind the discovery... astronomy moderators should have either closed or migrated to physics. $\endgroup$
    – hjhjhj57
    Commented Jun 22, 2015 at 0:32

1 Answer 1


The very fact that light has finite speed was first discovered in astronomy. See for example the Wikipedia article on Aberration of light. Light which is responsible for this phenomenon travels in the empty space. By the way the difference of speed in the air and in vacuum is small, and the first precise measurement of this speed were made in the air.

To conclude from these measurements what is the speed in vacuum is easy by combining with an astronomical observation. Indeed, the difference of light speed in vacuum and in the air is responsible for the phenomenon which is called (astronomical) refraction. And refraction of the light coming from a star can be very exactly measured. If you know the speed of light in the atmosphere, you can compute the speed in vacuum. This is the general principle.


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