It is the same principle at work in a Spider's Web. You can't see the web, what you see is a little light being reflected off the web. The web itself is transparent. But a spider's silk is much more flat than round, which causes light to be refracted, and then reflected. If there is more light being reflected from behind the web than there is being reflected off the web, the web appears to be invisible, and this is almost always the case.  THAT'S WHY IT WORKS!