Equal or unequal
Which arc is bigger? At first glance, the upper appears smaller than the lower. If you change their order, their sizes seem to change as well.
In fact, both are the same size. This can be verified by placing them on top of each other. The fact that they nevertheless appear still larger or smaller when placed side by side is due to an optical illusion, the so-called Jastrow's illusion.
Further informationson the exhibit "Equal or unequal"
Jastrow's deception arises from the fact that people do not perceive objects in their actual size, but always relate them to their surroundings. So we are not able to determine the absolute size of the arc. Instead, we compare the lengths of the circular curves. Because the outer curvature of an arc is always larger than the inner curvature, the upper arc appears smaller.
We need a third of our cerebral cortex only to create these images. In order to reduce the computing power, our brain often simplifies. To reduce the processing power, our brain often simplifies the image by comparing only the lengths of the facing lines and not the two bodies. Since one is shorter, our brain assumes that one object must be smaller and constructs the image of objects of different sizes.