In optics, the curve formed by light rays reflected from a curved mirror or surface, creating a bright envelope of light.
From Greek kata- 'down' plus caustic (from kaiein 'to burn'), referring to lines of light that appear to 'burn' or concentrate when reflected. The term dates to 17th-century mathematical optics.
Catacaustics are why the bottom of a cup in sunlight shows a bright curved line—light bounces off the curved surface and concentrates along a mathematical curve that's been studied since the time of Newton.
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