On my recent commercial flight to St. John Newfoundland, I witnessed a very interesting optical effect. We were flying in the bright sun above the solid cloud layer. As we descended closer to the clouds, i watched as big circular rainbow appeared to flying right next to the plane. As we descended closer and closer and right before we dipped into the clouds, a shadow of the plane appeared right inside that rainbow.
I was so mesmerized that I did not even think about getting up and pulling my camera (that was in the overhead bin), so I could only tell people about it and I could not see they did not quite believed me.
So, on my return flight to Toronto (again in the bright sun above cloud deck), I flew with the camera in my lap, just in case it would be repeated again, and guess what - it did.