caustic

Definition: Luminous curve of light often seen in a glass or cup of liquid either near the surface of the liquid, reflected onto the side of the container or projected onto a surface. * Formed by the intersection of rays of light (i.e. it is the locus of such intersections) which cross each other due to spherical aberration: rays near the centre converge less strongly than rays near the edge of the lens. * Occurs in reflection as well as refraction of light.

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