– Michael Pickard and Gurpreet Singh: “The Windmills of your Mind Illusion”. The University of Sunderland. UK
Author description: How is it possible to see simultaneous forward and backwards rotation in the windmill when it is turning at constant speed and direction? Key factors in this illusion include: symmetry in the windmill design; the amount of rotation per frame; persistence of vision; and the propensity in vision to group together things that are in close proximity. The schematics provided show how differences in pitch together with the careful positioning of the windmill arms determines how visual groupings can be formed one way or the other and it is these that determine the direction of motion seen.