A caustic curve is the envelope of rays of light reflected (or refracted) by a given curve
from a given point source of light. A catacaustic results
from reflection, a diacaustic from refraction.
Among the caustic curves of a circle are limaçon, if the light source
is nearby, a nephroid, if the source if
at infinity, and a cardioid, if the source
is on the circle.