Climbing Sphere Illusion
How we did it:
Gather your materials. Then, bend half of the first ring of the slinky up so that it forms the shape of a hook and tie one end of a piece of string to it.
Tie the other end of the string to a tree branch.
Push the ball into the twelfth ring of the slinky.
Twist the string in one direction. Then, let go and observe what happens as the slinky spins in the opposite direction!
What’s going on here is an optical illusion. Optical illusions happen when your eyes and brain get confused by something you’re seeing, so the thing appears differently to you than it really is. And actually, there are two optical illusions at work here, so you’re doubly confused!
The first illusion is called a Barber Pole Illusion. That’s what makes the slinky look like it’s moving up and down when it spins. Your eyes have trouble tracking the spinning motion, so instead, your brain is tricked into seeing the coils moving vertically. The same thing happens with a barbershop pole.
The second illusion is called Induced Movement, and it’s why it looks like the sphere is moving. Because the slinky looks like it’s moving, your eye starts to follow that motion. Anything that doesn’t also move with that motion (like the sphere) appears to move in the other direction! Focus on the sphere, though, and the illusion will stop.