The breathing rectangle
Look at the still disk, then start rotating it and slowly increase the rotation speed. What can you see? Experiment with different angular velocity.
With the adequately adjusted small rotation the speed you get an impression that the sides of the figure move and the whole shape distorts. The effect gets stronger after you look at the rotating disk for a while.
How it works
Man can differentiate shapes better than colours. The human eye follows lines which enables shape analysis. The figure on the disk consists of rounded rectangles; parts of them are straight lines captured by the eye the most quickly, other parts are semicircles. The eye follows the lines and comes to the semicircles which distort the shape in of its circular movement – this particular distortion is perceived by the brain as undulation.
The phenomenon of following the lines by the human eye is often used in photography, films, painting for three-dimensional effects.