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When I was a kid growing up in Philadelphia, I spent a great deal of time at the Franklin Institite Science Museum. They used to have a 2nd floor exhibit called the Hall of Illusions. It wasn't fancy or hi-tech, but I was fascinated by optical illusions. I still am.
Fast forward to teaching math. Whether you are teaching in traditional 45 minute periods or in block scheduling, you are responsible for coming up with lessons that fit a given amount of time. Sometimes you misgauge it and are talking right to when the bell rings and at other times, you finish early.
When most teachers finish a lesson a bit early, they typically let students start on their homework. Some students do but other students figure the lesson is over and start to talk among themselves. It always seemed that when this happened to me, an administrator would walk in and of course, question why I wasn't teaching.
So I decided to put the two situations together: my love of optical illusions and having a little time left in the period. I created a set of Powerpoint slides that illustrate 64 optical illusions. It is something that you can show with a couple of minutes left in the period and will fully engage the students. If you have a projector in your classroom and Powerpoint on your computer, you can pull these out anytime you have 3 or 4 spare minutes and show one. It will leave the kids wanting more.
Some of them are old standards but many are relatively new (there is a contest held every year for new illusions and I have included many of the best of them). Some involve animations. Some are explained in terms of why the illusion occurs while many do not because the explanation is either too complex or not fully understood. But all of them are captivating.
They are absolutely free. Just download them from the list below. Depending on your computer setup, the Powerpoints may go into a download file and you can open them from there. If you have a Mac and use Keynote, you should be able to open Keynote and then open the files from the file menu. They should work in Keynote as well.
For memory purposes, they have been divided into 12 slide shows, each containing as few as one slide or as many as 10. None of these illusions are mine but are based on Internet sources on the Internet and I supply those sources at the end of each show.