Here we see two curves of different shapes inside a ring. On rotating this, we see a recognisable bell shape.
This is because our brain combines a sequence of continuously changing images into a single image.
The exhibit here has metal rod bent in the shape of two irregular curves. These are fixed to a ring that can be rotated about a vertical axis with the help of a shaft that is provided.
On rotation, we no more recognise those irregular curves but, we begin to see a familiar bell shape even though the rod is not at all bent in that shape in reality.
This is similar to the effect of seeing a spherical shell kind of a structure when a bangle or any circular frame and even half a portion of it is set into rotation about a vertical axis.
The two irregular curves are actually portions of the same familiar bell shaped figure in two dimensions. But they are chosen in such a way that same part of it is to the right side of axis and the remaining part is to the left side of the vertical axis. Also, if we imagine the mirror images of those two curves, then the actual curves together with their mirror images would give the complete bell shaped figure.
It we actually use mirrors; we would see the bell shape but only in one particular plane.
But, the bell shape we see here on rotation is not restricted to a single plane. It can be seen from all around. It is as if having the mirrors in the appropriate positions that are suitable for the person’s line of sight. This can be explained as follows :
We actually see any object when the light coming from the object (either its own or the reflected light) reaches our eyes and forms an image on the retina. The image formed on the retina is retained by the brain for some fraction of a second (~1/5th of a second) this is known as persistence of vision.
As we set those curves into rotation, their position changes depending upon the speed of rotation.
That means the corresponding images of the curves on our retina also changes accordingly. So, there is a sequence of continuously changing images on our retina. But because of the persistence of vision our brain puts them all together into a single image.
Hence, we end up seeing a bell shape in this exhibit.