Many thanks for the links, Quest, but they don't answer my problem. I was looking up at a sky (at approx 45 degrees(?) which showed a "sunbeam" against a part of the sky which had no clouds in it. It's difficult to describe, but part of the sky was (when looking up at it) clear(ish) blue. The sun was shining both on that part of the sky and through the cloud. Looking (at the clear part) of the sky, there was a very distinct "sunbeam" (IE, a change of colour/brightness, fanning out and fading with distance) from the hole in the cloud. I don't understand how this could occur. But I witnessed it... Sunbeams against occluded backgrounds I can understand. But how can physics explain a "shaft of light" from the same light source against the same background and through the same medium? My 1st theory is: The hole in the clouds wasn't 100%. Thus the light there got reduced. So the light in the sky was affected by that (in some way). But I'm in no way convinced that that's the whole answer. Still baffled... Cheers, Diolma