La Cassiopea & Caustics ~ Antonino Gumina imaged "La Cassiopea" (Cotylorhiza tuberculata) near Capo D'Orlando, Messina, Sicily.   The surrounding bright lines and patterns are caustics playing on the shallow sea bed and visible through the crystal clear Mediterranean water.   ©Antonino Gumina, shown with permission.

Caustics are Nature’s ‘focussing’, quite different from that laboriously achieved by our multi-element lenses.    Caustics play and dance. They are ever moving and yet are always sharp with strangely permanent and stable forms that we recognise and yet find hard to define.    In contrast our lenses are fragile, they lose their focus with the slightest shift of an element or the CCD plane, their focus is unstable in the sense that caustics are robust and strongly stable.   

Caustics mark spatial discontinuities in light ray behaviour.     The gently undulating sea surface refracts sun rays.   Regions where the surface is convex deflect rays  to form volumes where pairs of rays cross.   At the volume edges the ray crossings cluster and there is a corresponding concentration of light – a caustic surface.   Elsewhere there is no rays crossing.   The caustic surface is at the boundary of two spaces where pairs of rays cross or do not cross at all.

The intersection of the caustic surfaces with the sea bed gives the bright dancing lines and patterns.   The colours largely arise from further refraction of light from these lines at the sea surface.