Red Dot Sights - How They Work
Encyclopedia of Bullseye Pistol's website has an excellent article explaining how red dot sights work, dispelling some common misconceptions about them, and discussing some of their shortcomings. This article is definitely worth reading in order to get a better understanding of these devices.
A while back I did a post on red dot sights covering some of the same topics, and touching on some others. You might want to give this post a reading too.....