The Ehrenstein illusion is an optical illusion studied by the German psychologist Walter Ehrenstein. The sides of a square placed inside a pattern of concentric circles take an apparent curved shape, however the square is perfect with perfectly straight sides. There are few simmilar illusion inside this post, and sometimes the name “Ehrenstein” is associated with one of the illusory contour figures. As you see, the ends of the dark segments produce the illusion of circles (inside this post). Eventhough the apparent figures have the same color as the background, they still appear brighter. Very similar effect is obtained in the Kanizsa triangle. All of them can be found inside this post, and if this isn’t enough for you, be sure to check “Seemingly Bent Objects Category” for more seemingly bent optical illusions.
