The Floating Marble

Illusions take place when our brain misinterprets information it receives from our senses. In this activity, you'll be making a simple decoration for your room, made from a wire coil and a marble. The decoration is also an illusion: when the coil spins, the marble appears to be moving up and down.

Materials

• One marble (preferably one that is 1.5 cm in diameter and a solid color)
• 60 cm of stiff, insulated electrical wire
• Thread for hanging the decoration
• A funnel
• Scissors

What to do

1. Wrap the wire around the neck of the funnel to form a spiral coil.
   
   
   
   
2. Make a loop at the upper end of the coil, and tie about 15 cm of thread to it.
   
   
3. Stretch the coil until the rings are about 2 cm apart from each other.
   
   
   
   
4. Place the marble in the space inside the coil.
   
   
   
   
5. Hold the thread and spin the coil in one direction, and then in the other. Does the marble rise and fall?

Something else you can do
Spin the coil at different speeds. How does that change the illusion? Make another coil wrapped in the opposite direction (so that one is wound clockwise and the other counter-clockwise).

Where's the science here?
When you look at the coil, you see only one side. The coil forms diagonal lines across the marble. When you spin it, the diagonal lines appear to be rising and falling (depending on the direction of the spin), even though the coil is rotating on its axis. The brain interprets what it perceives as the movement of the marble, rising and falling (in the opposite direction of the spinning diagonal lines).

We perceive the location of objects in relation to each other. Therefore, we perceive the location of the marble in relation to the lines that form the coil. The movement of the diagonal lines across the marble can be explained in two ways: either the coil is spinning upward or the marble is moving downward. The brain prefers the interpretation in which the marble moves, since it is evident from the lines above and below the marble that the coil is spinning rather than moving up and down.

This illusion is similar to two others in the exhibit: the sound illusion, Forever Upwards and the Endless Staircase.