Optical illusions are cool! They reveal that the brain does not rely only on our eyes for perception but also on our knowledge about how the world operates. One can learn the ‘rules’ that the brain uses to formulate perceptual experiences by studying how it can be ‘tricked’. Below are some examples of geometrical illusions. ‘A’ and ‘B’ are always either the same size or flush with each other.
Delboeuf illusion: In this geometrical illusion, the yellow circles ‘A’ and ‘B’ are identical in size but ‘B’ appears larger. We perceive the inner yellow circle as being larger in size as the surrounding pink annulus is closer to it.Ebbinghaus illusion: The Ebbinghaus illusion is similar to the Delboeuf illusion. Again, the yellow circles ‘A’ and ‘B’ are identical in size but ‘B’ appears larger. We perceive the inner yellow circle as being larger in size as the surrounding pink circles are smaller and closer to it.Ehrenstein illusion: In this geometrical illusion, the yellow square appears distorted. Even though the four sides of this square are of equal length, the ‘A’ side appears longer than the ‘B’ side.Helmholtz square illusion: This geometrical illusion consists of a perfect square of horizontal stripes. Although the four sides of this square are of equal length, its height ‘B’ appears taller than its width ‘A’ is wide. This illusory effect runs contrary to the popular belief that clothes with horizontal stripes make us look fat.Horizontal-vertical illusion: The horizontal-vertical illusion (a.k.a. the vertical-horizontal illusion) consists of a vertical line ‘A’ rising from the center of a horizontal line ’B’. Although both lines are of equal length, the vertical line ‘A’ appears longer than the horizontal line ‘B’. The illusory effect is less prominent if the vertical line is not at the center of the horizontal one.Jastrow illusion: In this geometrical illusion, two Pacman-like shapes of equal size are presented one on top of the other. Although both shapes have the same size, the one on the bottom ‘B’ appears bigger than the one on top ‘A’.Müller-Lyer illusion: This geometrical illusion consists of arrows pointing either inward ‘A’ or outward ‘B’ on both ends. In this example, the shafts of the arrow-like shapes are of equal length yet shaft ‘A’ appears longer than shaft ‘B’. Interestingly, right-angle corners are generally man-made and there have been demonstrations of indigenous tribes living in circular huts being less susceptible to this illusion.Oppel-Kundt illusion: In this geometrical illusion, the physical center of the figure is the second line from the right yet the left half of the figure ‘A’ appears wider than its right half ‘B’.Poggendorf illusion: In this geometrical illusion, a continuous line passing behind a barrier will appear displaced. In this example, ‘A’ and ‘B’ make up a continuous yellow line behind the pink barrier yet they appear to be displaced as if they were two different lines.Ponzo illusion: In this geometrical illusion, the upper yellow horizontal line ‘A’ appears longer than the lower yellow horizontal line ‘B’. Both lines are in fact of equal length and project identical images on our eyes yet the brain will compensate shrinking of what we see in the real world with increases in apparent distances.Sander’s parallelogram: In this geometrical illusion, the yellow diagonal line ‘A’ bisecting the larger left pink parallelogram appears to be considerably longer than the yellow diagonal line ‘B’ bisecting the smaller right pink parallelogram. Yet, both yellow diagonal lines ‘A’ and ‘B’ have the same length.Shepard illusion: Also known as the ‘Tabletop illusion’, tabletop ‘A’ facing away from us looks longer and narrower than tabletop ‘B’ that is facing sideways. Yet, both tabletops ‘A’ and ‘B’ are identical.Square-diamond illusion: In this geometrical illusion, a square in its diamond position ‘B’ appears larger than its standard position ‘A’.
Philippe Chouinard - Vision & Action Lab 