Egret and Fish with Caustic Network

Provided and copyright by: Mila Zinkova, Fogshadow
Summary authors & editors: Jim Foster, Mila Zinkova, Stu Witmer

This egret is about to snack on an unfortunate fish. Unnoticed to either bird or fish, and to most people as well, is the faint caustic network reflected on the chest of the egret -- blotches of sunlight and shadow sometimes called "glitter lines". Undulating water surfaces can act like an arrangement of concave and convex lenses, which results in an uneven brightness distribution at the bottom of a shallow sea floor. These curious networks are commonly seen in pools. The height of the waves determines the focal length of the formed "lenses." Caustic networks are most easily observed when the water's depth is about five times the wave's crest-to-crest distance.