Pulfrich Phenomenon demonstrated with GeoGebra applet.
The Pulfrich phenomenon or effect is a psychophysical perception related to the apparent spatial disparity between the two eyes when the sight of one of the eyes is purposely passed through a filter. The effect was observed in an experiment with a swinging pendulum that was first performed in 1922 by a German physicist called Carl Pulfrich. The pendulum is swung back and forth and its movement is observed with both eyes. Care must be shown that the movements of the pendulum stay in the same plane parallel to the viewer's forehead, who is making the observation.
When viewed normally with both eyes, the pendulum just seems to swing back and forth. When a filter is placed in front of one eye, the pendulum suddenly seems to be swinging in an ellipse parallel to the floor. In other words, the pendulum should appear to be tracing an oval path. This is an optical illusion. Dimming the light equally to both eyes does not cause the illusion.
You can observe the phenomenon by downloading the ggbfile and clicking at the slider t Animation On.