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Homework Help: Vacuum, index of refraction

  1. Apr 26, 2007 #1
    can you please explain some of these questions to me? Thanks.

    In the PR book, it defines vacuum as empty space and I don't really know what that means. Does it mean that there is nothing (no air molecules, no other gasses, etc.) in that space? (is the atmosphere of space a vacuum also?) I've come across this term in the optics section and i think I remember seeing it in the fluids section. There's no pressure in a vacuum right?

    I found a problem on page 312 # 20 that involves a vacuum and I think I used the atmosphere pressure instead of no pressure at all for P_o. So... vacuum is just empty air or does it need to be when an acutal vacuum machine is actually sucking air up? And like when the lids of containers are sealed, do you use vacuum pressure (0) for the P_o inside unless otherwise stated?

    Also, just to make sure, the speed of light travels the fastest in a vacuum right? It can never travel any faster in any other material? This will help me remember the index of refraction more because I think the equation is weird since it has the speed of light on the top and not the bottom. Thanks a lot!
  2. jcsd
  3. Apr 26, 2007 #2


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    Yes, a vacuum means that there is absolutely nothing there, no air molecules, no gases of any sort. Note that while sound cannot propagate in such conditions, light can.

    You ask whether the "atmosphere" of space is a vacuum. Can you better define what an "atmosphere" is?

    You ask whether there is no pressure in a vacuum. Can you better define what pressure is?

    What do you mean by "empty air"?
    When the lids of containers are sealed, there is no reason to assume there is a vacuum inside unless one is created (ie. by sucking the air out). Do you think your cookies experience a vacuum every time you put on the lid of that jar?

    Light travels at its fastest in a vacuum, and that speed is known thus as the "speed of light in a vacuum". In any material, light will travel slower as all natural materials have an index greater than 1.
  4. Apr 27, 2007 #3
    thanks for your help!
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