Caustic rays produced by sunlight below the surface of seawater. The rippled surface of the water acts like many small lenses, focussing the light into beams. In optics, a caustic or caustic network is the envelope of light rays reflected or refracted by a curved surface or object, or the projection of that envelope of rays on another surface. These curved surfaces act like assymetric lenses focussing the light rays into constantly changing patterns.