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E=mc2 question

  1. Aug 16, 2011 #1
    i know this might be a dumb question but it was in my head and i would love to hear an answer,

    so the famous E=mc2..e obviously energy, m mass, c is the speed of light,
    but what speed of light? we all know that c is constant at vacuum but it changes when it goes through another medium like air or glass or whatever it is, so my question was when scientists actually apply this equation...what c do they use? like do they use c of air for example or d they only use the normal c which is 299,792,458 metres per second?
  2. jcsd
  3. Aug 16, 2011 #2
    "c" always refers to "speed of light in a vacuum"
    so there's no confusion here.
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