The ring-segment illusion
Yes, the two train track pieces are the same lengths. I know this because we have a basket of toy train tracks, and I took two identical curves and lined them up for the outline of this sketch. But it's easy to be fooled and think the one at the top is shorter than the one at the bottom.
While there may be other factors to help explain the ring-segment illusion, a common one is that our brains compare the short inner side of the top track against the longer outer side of the lower track leading us to believe the top one is shorter. Aligning the top track to the outer left point of the lower track enhances the illusion by increasing the horizontal contrast.
I like the ring-segment illusion as the name for it. It's also known as the Jastrow illusion after the Polish-American psychologist Joseph Jastrow, also known for the ambiguous figure of the rabbit-duck , who published a tapered version of the ring-segment illusion. In his paper, he gives a related example of how a square sitting on a point can look larger than one sitting on a side as we subconsciously compare the longer diagonal length with the shorter side length. You can test that in the sketch.
