Modern Physics: Plasma Tube / Lights and Atomic Spectra / Cosmic Rays / Spirograph / Probability Board
The Spirograph
 

The exhibit is composed by a laser, two mirrors attached to two motors. A third mirror is used to project the images on the ceiling.

When the laser beam strikes the first mirror-motor component, it projects a circle of light of variable frequency. When both mirrors are spinning, the two circular patterns superimpose to produce images known as spirograph. The spirographs are mathematical curves of the variety technically known as hypotrochoids and epitrochoids.