Obligatory Warning.......
What you are about to read should be taken more along the lines of semi-informed interpretation. The truth of the matter is no one knows for a fact everything about propagation and all the factors in creating it. These are all things that I've learned and hopefully interpreted correctly. I may be wrong and if I am please let me know. I do not claim to be an expert in these things, but like most people, I want to learn.
(Or Lasers)