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How does the Rubber Pencil Illusion work?

Remember how in school you used to perform magic by making a solid pencil turn into rubber by simply jiggling it up and down? Let’s understand the basic science behind it.

To begin with, your eyes and brain don’t follow the same pace. When light enters your eye, receptors like rods and cones send signals to your brain to process the information through neurons. Now think of these signals as pictures. When the brain processes this information it goes through all the pictures just like when one goes through a flip-book.

But the visual system of humans is remarkably slow. We are only able to process about 50-100 pictures per second. Whereas birds can process about 145 pictures per second and flies, 270!

So when you’re viewing a fast moving object you don’t actually see the object move in real time. Instead each picture leaves a millisecond long impression on the retina. So when someone jiggles a pencil in front of you, what your visual system picks up is a summary of the pencil’s movement and not the details of it. When you join all the pictures together you see a curve.

But if the same is viewed by birds or flies, due to their ability to process images faster, they see a straight line and not a curve.



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