Thaumatrope

Can you see both sides of the disk at the same time?

Spin the disk and focus on the image.

When the disk spins fast enough, you see the images on both sides of the disk in such a rapid succession that your brain combines them into a single image.

Since the previous image remains in the retina and brain for a short time, there is time for a visual perception of the following image to be created in the brain at the same time. As the spinning continues, you now see both images continuously superimposed, although they are actually on opposite sides of the disk and are not, in reality, simultaneously in our field of vision. This phenomenon is called an afterimage. All of the moving images we observe are based on the afterimage phenomenon. The thaumatrope or “wonder turner” was introduced in England in 1825.

A strong afterimage can be experience by staring at a source of light and then turning to look at a light surface. After about one second, the afterimage turns into a “negative”, or, in other words, its complementary colour. The afterimage may, in some cases, be irritating, but usually we do not pay any attention to afterimages.