Nephroid
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In geometry, a nephroid is a specific plane curve. It is a type of epicycloid in which the smaller circle's radius differs from the larger one by a factor of one-half.
Nephroid caustic at bottom of tea cup
Caustic (optics)
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In optics, a caustic or caustic network is the envelope of light rays which have been reflected or refracted by a curved surface or object, or the projection of that envelope of rays on another surfac
Nephroid
…108 a 4 y 2 onto the curve of degree 6 with equation ( 4 a 2 − ( x 2 + y 2 ) ) 3 = 27 a 2 ( x 2 + y 2 ) y 2 (see diagram) . A nephroid in daily life: a caustic of the reflection of light off the inside of a cylinder. References Arganbright, D., Practical Handbook of Spreadsheet Curves and Geometric Constructions…
Caustics from a glass of water
Caustics made by the surface of water