The Spiral

What you can do here
  1. Look at the pattern on the still disk, then start rotating the disk and gradually increase the rotation speed. What can you see? Try rotating the disk with different angular velocity in both directions.
  2. Keep looking at the rotating disk for about 30 seconds. Then quickly look at an unmoving object nearby (or look at your feet).

How it works
  1. The rotating disk makes an impression of a spiral moving towards or away from the observer (depending on the rotation direction).
  2. After looking at an immobile object for a while you get an impression that the observed object is undulating.
This is due to the physiology of vision. Lines of different thickness and intertwined colours create a 3-D illusion.
If you look at a disk for a long time, you get a specific afterimage called the waterfall illusion. The brain recognises the direction of movement with the help of two receptors, each responsible for one direction. When you look at motion in a specific direction, an appropriate receptor sends a strong signal to the brain. Similar signal strength coming from both receptors inform you that there is no motion. When one of the signals continues for a length of time, then even after it stops, it takes the brain some time to recognise them signal from the other receptor and regain the state of equilibrium.

Interesting facts
The waterfall illusion was described in the early 19th century. If you look at falling water for a length of time and then quickly shift your eyes towards unmoving rock you get an impression that the rock moves upwards. The intensity of this impression depends on how focused you are, if you get distracting stimuli, the observed effect gets weaker.