Breathing rectangle

HOW

1. Look at the stationary disc.
2. Spin it slowly, increasing the spin speed. What can you see?
3. At low spinning speed, you get the impression that the sides of the figure ripple and the whole figure is deformed.
4. The observed effect becomes more apparent after gazing at the spinning disc for a while.

WHY

Humans are much better at distinguishing shapes than colours. This is because the human eye follows lines. Meanwhile, the figure on the disc is composed of rounded rectangles. Part of each figure consists of straight lines, caught by the eye the fastest, and part are semicircles. The eye follows the lines and encounters semicircles that disrupt the regularity of the shape in rotational motion – the brain perceives this fact as rippling.