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
they have 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.
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 mind.