Alan Clark took these photos during a flight across Canada.
If you have a window seat, the high altitude skies provide many optical effects. There are glories, opposition glows, above and below horizon ice halos, flattened suns and moons, long shadows and extraordinary dark and purple skies.
Alan also found plenty of optics inside the cabin to help while away the hours.
Image ©Alan Clark, shown with permission
Yet more caustics.
Alan watched the ever changing sun's reflection from the orange juice projected onto a seat back. Aircraft vibrations produced yet more caustics, more reflection caustics. Their appearance suggests occasional concentric ripples on the juice surface.
Pure concentric ripples are not easy to generate - watch this re-creation of those made by T-Rex in Jurassic Park.
Orange Juice Caustics
The juice is lit by two curves met at a cusp. These are caustics, sharp surfaces of bright light formed where there is a high concentration of rays. Sun rays reflect from the plastic cupís curved surface before splashing on the juice surface. A sunlit coffee or tea cup has them too.
At right: A computed ray diagram. The caustics are tangent to each reflected ray and mark the location where the ray concentration is greatest.