Plural of caustic; caustic substances used in chemistry, or in optics, the envelope of light rays reflected or refracted through a curved surface.
From caustic, from Latin causticus, Greek kaustikos (burning). The plural form is used both for chemical substances and for geometric optics phenomena.
In optics, caustics are those shimmering light patterns you see at the bottom of a swimming pool—they're called 'caustic' because they look like fire, and the mathematical study of them fascinated medieval scholars and Renaissance artists.
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