I wasn't sure what to expect, but wow this was really really cool.<p>I have a reasonable understanding of optics but I'm still trying to understand how this works. I'm looking at this explanation but still trying to break it down:<p><a href="http://www.ian.org/Schlieren/HowTo.html" rel="nofollow">http://www.ian.org/Schlieren/HowTo.html</a><p>Anyone have a good intuitive explanation?