1. Observe the pattern when the disc remains immobile. Start rotating the disc. Gradually increase the rotation speed. What can you see? Try rotating the disc with different angular velocity in both directions.
2. Keep looking at the rotating disc for about 30 seconds. Then quickly direct your eyes to some object not far away and not moving (or look at your feet).
How does it work
1. The rotating disc makes an impression of the spiral moving towards of away from the observer (depending on the rotation direction).
2. After looking at an immobile object, for a while there is an impression that the object being observed is waving.
The presented results are connected to the physiology of vision. Lines of different width and intertwined colours allow to create an illusion of depth.
Prolonged gazing at the disc causes a specific afterimage called the waterfall illusion. Recognising the direction of motion is possible due to two receptors – each responsible for one direction. When we observe the motion, the appropriate receptor sends a strong signal to the brain. The equal signal strength coming from both receptors informs us that there is no motion. When one of the signals continues at higher lever for a longer period of time, then even after it seized it takes some time for the brain to acknowledge the signal from the other receptor and regain the state of equilibrium.
The waterfall effect is the name of the illusion of rock moving up while intensely gazing at the water falling down.