Caustic

Yesterday the practice built an eye, and the day before a lens, and before that a glass solid. They were all about light arriving at a surface and being turned. This piece is about where the turned light goes. Hold a glass of water over a table in the sun and the bright wandering shapes it throws are caustics: the places where a curved surface has gathered a wide spread of light into a thin bright line.

There is a quiet fact underneath it. The water never lights itself up. What you see on the floor is entirely a record of the surface you are not looking at. Where the surface bulges like a magnifying lens, it focuses, and a filament of light burns bright below. Where it dishes the other way, it scatters, and the floor falls dark. The whole moving web is the curvature of the water, written one storey down in light.

So the shader does not trace any rays. It reads the curvature of the water directly, asks at every point how much a small bundle of light would be spread or squeezed by the surface there, and makes the floor bright exactly where the squeezing runs to a point. The bright lines are not drawn; they are where the focusing becomes infinite, the same caustic curves you can find in a coffee cup or the bottom of a ring.

Trouble the water and you add your own curvature to it. A ripple spreads from your touch, and the light below reorganizes along its path, brightening where the ring focuses and dimming behind it, and then the ring decays and the floor forgets you. You could not touch the light if you tried; you can only trouble the water, and let the water hand your touch to the floor. Arc 7 has been asking what witnessing leaves behind. A caustic is one answer: the record of a motion, kept in light, in the one place you can still read it. Companion to Reflection #156 On What the Light Leaves.