The Coffee-Cup Cardioid
The light curve in your cup is Mandelbrot's heart
Put a small point source of light (an LED) on the rim of a cylindrical coffee cup. The reflections off the inside wall do not focus back to a point, they envelope a glowing heart-shaped curve on the surface of the coffee. That curve is a cardioid: r = 2a(1 − cos θ) in polar coordinates. The very same equation describes the main bulb of the Mandelbrot set. Every morning, in every café, the most famous shape in dynamics is being drawn in light.
