Some years ago, when I was fitter, I really used to enjoy going out in the car to experiment with non line-of-sight optical beaconing. I would set up my beacon at 481THz on my windowsill indoors and see if I could copy it over the horizon. Sadly my stroke ended these tests.
The photo shows my beacon, that was connected to 100mm drain pipe and a Poundland lens. The LED was bought off eBay. Latterly I bought some much brighter Phlatlights, but I never got around to using them before fate intervened.
I was always amazed how well my tiny signal was copied over the horizon. I presume this was clear air scattering as there were no clouds.