Breathing rectangle

What can you do here
Observe the disc when it remains static. Start rotating the disc. Slowly increase the rotation speed. What can you see? Experiment with different angular velocity.

With adequately adjusted, small rotation speed you get an impression that the sides of the figure move and the whole shape distorts. The effect gets stronger after staring at the rotating disc for a while.

How does it work
Man is more capable of differentiating shapes than colours. Human eye follows lines, which enables shape analysis. The figure on a disc consists of rounded rectangles – some of its parts are straight lines, which are caught by the eye the most quickly; other parts are semicircles. The eye follows the lines and encounters the semicircles, which distort the shape during its circular movement – this particular distortion is perceived by the brain as waving.

Interesting facts
The phenomenon of following the lines by the human eye is often used in photography, filming, painting, and three-dimensional arrangements.