
Caustics also appear within eyes when the direct sunlight travels diagonally through the cornea. Blog reader P.A. Farris sent me these photos she took of a black-crowned night heron. The caustic is an arc-like shape on the opposite side of the pupil from the highlight.

If the light comes from far enough to the side, the caustic becomes a focused point.
Thanks, P. A. Farris
More on caustics in Color and Light: A Guide for the Realist Painter
Previously on GJ:
Caustics
Nephroid Caustics
Caustic Reflections
Wikipedia on caustic optics