In our discussion of camera resolution, we described how a point source appears, not as a point, but as an Airy disk in the camera.This phenomenon is, in fact, fairly ubiquitous. An interesting variation is to see the Airy disk projected by the sun illuminating an airplane and projected onto the clouds below. I am not sure that I fully understand the mechanism here. But I thought
that I would show some examples that we took while flying into Minneapolis/Saint Paul for a recent experimetal trip to the Mayo Clinic in Rochester, MN. Figure 1 shows the disk itself, a bright central region and a ring progressively blue to red. In some cases you can see a second ring. Also note that the central peak is fairly weak, presumably because we are looking at a dark hole rather than a bright spot. I have also noted that, when the clouds get close, you often see the shadow of the plane centered in the ring (see Figure 2).
So I put it out for readers’ comments on the exact physical mechanism at work here. Judging by the angles involved and the order of the colors it is pretty clearly a diffraction phenomenon and not a water droplet or ice crystal phenomenon like rainbows and solar halos.
Whether or not you are interested in the physics, keep an eye out for this next time you fly. The can be very dramatic and cool. In the meantime, I am going to do some furtehr research and see if I can come up with a more complete explanation.