How good are your eyes?

I teach physics which often leads me into teaching a lot of off topic ideas such as optical illusions during the teaching of my optics unit.

In my quest to stay ahead I was researching material to use to teach optic after the Christmas break and I found this.

And then of course I had to find out why it worked this way so thank you very much wikipedia.

Everything comes down to physics or stamp collecting.

Enjoy.