Caustics from Radiants at Finite Distance Generated by Rolling Circles
Light from a radiant point (red) reflects off the blue curve. The envelope of the green lines of reflection is a caustic curve (purple) that can be seen as the concentration of the green lines. Boyle's theorem states that the blue curve has a corresponding red curve and a circle of varying radius that rolls along the red curve such that a point on the rim of the circle sweeps out the caustic.
Click the image to move the location of the radiant. With "snap to" selected, orange dots appear at radiant locations of some interest and the radiant snaps to the nearest such point.