Caustics

I have been fascinated by caustics for a long, long time. I still remember the first time I noticed them – a bright, ethereal form dancing in the shadow of my mother’s wine glass. I was entranced by the way the light moved when the wine swished in the glass, and disappointed when my usually all-knowing mother wasn’t able tell me anything much about them.

Many years later, a friend asked me if I happened to know anything about caustics; I had never heard of them, so she explained that she was talking about the shifting patterns of light made by rippling water, the curves of light you see at the bottom of mugs, and so on. Finally I had a name for these patterns that had enchanted me since my infancy; when I got home I looked up the word, wondering what these things might have to do with caustic soda or holocausts.

Most caustics are quite harmless, but if you have ever used a magnifying glass to focus the light of the sun into a tight point to make smoking holes in things, you have witnessed their potential destructive power; this is where they get their name. Archimedes is famously said to have used a giant parabolic mirror to set fire to Roman ships using reflected sunlight, during the siege of Syracuse in 212 BC. In modern times, the Olympic Torch is similarly lit by a large parabolic mirror focusing the sun’s rays on a single point.

Caustics can occur whenever light leaves a curved surface; most often that means it has been reflected or refracted. Refraction caustics, caused when light rays are bent by passing through something, tend to show less extreme distortion than reflection caustics, but often show subtle colour variations like light from a prism, because shorter wavelengths of light are refracted more than longer ones.

Either kind of caustic can hugely amplify tiny imperfections or very subtle curvature into striking patterns, the effect increasing with distance from the surface. For example, very few windows are truly flat, and it is common to see cross-like shapes or mottles reflected on the walls opposite, when the sun is low in the sky.

Strictly speaking a caustic is the entire envelope of light which leaves a curved surface; the patterns of illumination we usually see are just the intersection of that three-dimensional structure with another surface. Something of the 3D nature of caustics comes out when the distance to the illuminated surface varies, with some features getting washed out with distance while others become ever more prominent. See this short video clip for an example; there’s a much longer film, with music, linked here.

We’d notice very quickly if they weren’t there – simulating realistic caustics is an important issue in computer graphics mainly for this reason, and an otherwise convincing scene will seem oddly flat and unreal if it is missing caustics that should be there. Mostly, though, caustics are one of those kinds of things which quietly make life that much more pretty while they just sit in the background, beneath our threshold of conscious attention – but which often reveal truly striking beauty when we pay them a bit of attention.

Additional photos courtesy of Reciprocity, SEngstrom and suchstuff; see more in the Caustics pool on Flickr. You might also like to play about with my interactive caustics-simulation animation, Zoobie.