# Coffee Cup Caustic

The light source is at C. Rays are reflected in the circle. Observe what
happens as you drag C inside the circle.

The light source is at C. Rays are reflected in the circle. Observe what
happens as you drag C inside the circle.

The phantom curve made up by the reflected rays is mathematically the *
envelope
* of the family of lines. This curve is called the circle
caustic. It is sometimes referred to as the *coffee-cup caustic* as it
can be seen reflected in the surface of a cup of coffee.

When C lies on
the circumference of the circle, the curve is a *cardioid*. If C is at
infinity, then the curve is a *nephroid*. From heart to kidney...

App generated by Geometry Expressions