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!