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Richard Feynman: no one has said it better..
Have you ever noticed the refraction of light in a cup of coffee?
The envelope of light rays reflected or refracted by a curved surface is called a caustic (from the Latin causticus, “burning”); more concrete a catacaustic in case of reflection, and a diacaustic in case of refraction. The caustic is a curve or surface to which each of the light rays is tangent. In case of a circle, the resulting shape is a cardioid.
Wolfram MathWorld provides a list of famous catacaustics.
Rainbows are familiar examples of caustics as well!