Underwater image by Daniel Perez at Papeete bay, Tahiti. Rays appear to be converging on the whale. Why? ©Daniel Perez, shown with permission.
|Daniel is looking downwards at the whales and oddly wavy shafts of sunlight converge towards the mother. We are seeing a roughly underwater equivalent of anti-crepuscular rays.
The 'rays' are not straight. They are caustics - sharp and thin sheets of light plunging downwards. The caustic sheets are produced by refraction of sunlight at the wavy sea surface. The caustics are stable in that the persist and also retain their sharpness even though the water surface is ever changing. The overall pattern changes but their form is preserved.
The sheets are more or less parallel and to an eye or camera looking opposite the sun direction they appear to converge by perspective to a point. The lower image shows the effect.
When you - or the whale - look upward there is a quite different sight. The surface is flashing with light. As an individual caustic crosses the eye we see two or sometimes three intense flashes of light on the sea surface.
At any point along a downward caustic sheet (a fold caustic) two light rays cross. We see them as bright spots on the surface. Sun glints reflected off water form the same way.
More about caustics.