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I was wondering if it possible to measure the pressure in a vacuum using light.

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For ideal gases at a constant temperature, we have that

$$n-1 = Ap$$

for some constant $A$. After calibration, measuring the refractive index of the gas using geometric methods should give you the pressure.

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    $\begingroup$ but then it wouldn't be vacuum... $\endgroup$ – ZeroTheHero Jul 14 '17 at 16:47
  • $\begingroup$ @ZeroTheHero Well, if you meant a perfect vacuum, then there's no gas, so there's no pressure. $\endgroup$ – probably_someone Jul 14 '17 at 16:48
  • $\begingroup$ yeah the question is ambiguous as stated, since it is not clear what pressure the OP wants to measure. $\endgroup$ – ZeroTheHero Jul 14 '17 at 16:49
  • $\begingroup$ And interferometric experiments can give you very high precision in measuring $n$. But there are some hitches. First, interferometry makes it easy to measure change in $n$, but you need an absolute reference to measure the value of $n$. And second the right reference is a complete vaccuum which is, well, hard to obtain. $\endgroup$ – dmckee Jul 14 '17 at 16:49
  • $\begingroup$ @ZeroTheHero I assumed the question was non-trivial and gave the answer accordingly. Arguments over semantics are not useful for practical questions. $\endgroup$ – probably_someone Jul 14 '17 at 16:51

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