Thaumatrope: An Optical Illusion
- Index card
This week, we will be doing an experiment on our brains! Or, more specifically, on how our brains receive messages from our eyes. We are going to make a traditional optical illusion called a “thaumatrope”.
Take an index card (or a piece of cardstock) and fold it in half with the unlined side out. Now, imagine two pictures on two pieces of clear plastic -- say, a bird on one, and a bird cage on the other. If these two images were put on top of each other, they would combine into one picture of a bird in a cage. (If you were to draw a polar bear on one and mountains on the other, however, when laid over each other you could see the mountains right through the middle of the polar bear, so that wouldn’t work.) On your index card, draw two pictures that could be combined if they were written on something transparent and layered on top of each other. I drew a fish and a fish bowl. For Fair St. Louis, we made fireworks on one side and the Arch on the other. If you’re not much of an artist, you could write your first name at the top of one side, and your last name at the bottom of the other. Be creative! Start out with simple designs with nice thick outlines, as these will be easiest to see.
When your pictures are done, slide a straw or coffee stirrer in between the two halves so that it looks like a rectangular lollipop. (Do NOT put the straw in the crease, like a flag.) Staple the straw in place as close to the middle as possible, making sure there’s a long enough straw to work as a handle.
Hold your newly made thaumatrope in between the palms of your hands and slowly rub your hands to spin it back and forth. You will see one side, then the other, then the first again.
Now, spin the thaumatrope faster and faster! What do you see? If all goes as planned, you should see the images combined, much as they would be had they been on transparencies laid on top of each other! This is an optical illusion because your eyes can only see one side at a time. It is physically impossible for light to bend around and show both pictures at the same time. However, in order for you to see something, your eyes have to send the message to the brain, and at some point your brain can’t keep up! Your brain fills in the gaps as best it can and blurs the pictures together, so you can see a fish in a fishbowl, a bird in a cage, or fireworks over the Arch.
Experiment with different designs to see what works well and what doesn’t. How fast do you have to spin it? Is it the same for everyone in your family?