Look at the first picture and it appears that there is a spiral formed by overlapping black arc segments.

Fraser spiral illusion 1

In the second picture below, when the arcs are highlighted in another colour , you can see that there is no spiral but a number of concentric circles.

The tilted tile elements in the picture cause your eyes to perceive phantom twists and deviations.

The Fraser spiral optical illusion was created by a British psychologist Sir James Fraser.