Very intriguing idea, Edley! Should be great fun experimenting. I'm going to look for some trial ray-tracing software...
Dave > Some time ago an idea for a nodus on a Large Sundial appeared to me. > It is the shadow of a thin-walled transparent sphere. This can be seen > simply in the shadows of soap bubbles and transparent round helium > balloons. This is not considering it a lens, just the shadow cast. As a > lens, such a structure is a diverging lens, but the shadow has a bright > spot > when the distance is proper for it's size. The central light bundle is > little > refracted and outer bundles are reflected at Brewster's or other angles, > leaving a > surrounding shadow. It appears much like the shadow of a disk with a hole > in it, but works the same with light from all directions. At a proper, > and quite large distance, the central aperture acts much like a pinhole. > I've not tried large spheres, but possibly those of glass or plastic, > especially coloured transparent materials would give a distinctive small > bright spot > surrounded by shadow. > I've no idea how to calculate the proper dimensions, size, wall thickness, > surface shinyness, etc. But, it could work very well at hundreds of feet > away. > If someone figures out how to calculate this effect, I'd be pleased to > hear about it. > > Thanks! > > Edley McKnight > > PS. John, I decided to let the list know. > > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > > --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
