Optical effects from the menisci of emergent rods in shallow water.

The shadow created by a narrow plant stalk or rod partially submerged in water is modified by refraction through the meniscus adhering to the plant. Using laboratory experiments and Monte Carlo simulations, we investigate the structure of the caustic formed by a vertical rod and discuss several aspects of it: the caustic's shape, brightness, and color; the "pinch" (distortion of the shadow); and the formation of a "dark halo" around it, all as functions of rod radius, water depth, and sun angle.