Caustics from a wine glass. Seen by Leo Van Hoyweghen
(site) in Belgium. Image ©Leo Van Hoyweghen, shown with permission.
The wine has refracted sunlight to form caustics on the bench. Look for them dancing on the bottom of a swimming pool when formed by the undulating water surface. The surface of coffee often has them, the result of reflections from the sides of the cup.
Caustics are sheets of light formed when rays bunch together, coalesce. They denote spatial discontinuities - catastrophes - in ray behaviour.
Rainbows are caustics, their bunched rays result form a turning point or angle of minimum deviation in the deflection of rays leaving raindrops.
More about caustics here.