# Caustics from Radiants at Finite Distance Generated by Rolling Circles

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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.

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Contributed by: Todd Will and Jeff Boyle (January 2014)

Open content licensed under CC BY-NC-SA