I am a Boeing 707 pilot and I see these rings on a regular basis whenever I go fly. It normally occurs on a bright sunny day when the shadow of my jet is cast on a flat blanket of clouds a few thousand feet below me. If the clouds are close enough, the outline of my jet can be quite crisp, but as the clouds become more distant the shadow will eventually disappear and actually become the brightest point in the middle of the colored rings. I always figured it is kind of like a magnifying glass/concave lens. If you put an object directly between a lens and the surface where its light is cast you can still completely eliminate the objects shadow when placed at the right distance. So, you may ask how does an airplane act as a lens? Well, it doesn't, it is like a prism. Since the light is cast all the way around my jet the result is a circle, much like a lens. And since the light is dispersed like it would be by a prism the resulting colors of violet, blue, green, yellow, orange, and then red can be seen. The colors in Prof Walter Lewin's picture aren’t so identifiable because of the poor surface of which he took the picture on. However, on a pure white cloud the colors are much more crisp. If you look very closely at the professor’s picture though, the colors are all there. No light other than the Sun's is used and no photographic tricks or special materials were implemented.