Caustic effects are those little spots or arcs of light that happen when sunlight shines through wavy water or a water-filled glass. The curving surface acts like a lens to focus the light into a small area.
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.
The same effects happen with human eyes. The light is coming from the right, and the caustic appears brightest as a curved shape at the edge of the iris. In the raking light on the iris, you can also see how the pupil bunches up.
Thanks, P. A. Farris
More on caustics in Color and Light: A Guide for the Realist Painter
Previously on GJ:
Wikipedia on caustic optics