Internal surface of a ceramic bowl
The internal surface of a ceramic bowl placed on blue paper and illuminated with sunlight forms a caustic—the curve where all light rays that have been reflected from a curved surface meet. Fujifilm XH-2, f/11, 1/3 sec, ISO-125, focal length 33 mm.
—Andrea Aiello, Max Planck Institute for the Science of Light, Erlangen, Germany