Materials Required
- Phonograph
- White poster board
- String
- Hole punch
- Large picture of sky
Optical Illusions
Working together , the eye and brain provide information about our environment.
But both eye and brain can be fooled, creating optical illusions. This project demonstrates how eye and brain interpret motion together and what happens when the perception mechanism becomes fatigued.
Spinning Spiral
Measure the phonograph turntables diameter. Tie one end of the string a little more than half that diameter to a pencil. Using cellophane tape, attach the other end of the string to the middle of the poster board Draw a circle by pulling the pencil tight against the string and swinging it around. Before you detach the string from the poster board, mark the center of the circle. Cut out the circle, punch a hole through the center mark and place this paper “record” on the phonograph turntable.
Switch the record player to 45 rpm or medium speed. As the record spins, hold a felt tip marker close to the center and slowly drag it out to the edge. You will create a perfectly drawn spiral. To draw a spiral in the opposite direction , flip the record over.
This time, begin at the edge and slowly move the marker towards the center.
Place the turntable 1.8m away. Tape the cloud picture to the wall ,and clearly illuminate it. If possible, increase the phonograph speed to 78 rpm. Look down at the turntable and stare at the spinning spiral for about 30 seconds. For 45 rpm, stare at the spiral for about 1 minute.
Now quickly look up and stare at the cloud picture , The clouds appear to move. This effect persists for 20 s even if u look away and back again. Notice too that the clouds seem to move in a direction opposite to that of the spin of the spiral. If the spiral moves inward the clouds appear to drift towards you. If you flip the record over and reverse the spiral’s direction , the clouds appear to move away.
What’s happening:
Receptors in the eyes work with the brain to detect inward and outward motion. When you look at stationary objects, inward and outward receptors are in balance. But when you look at a spiraling pattern, the stimulation makes one set of receptors tired. As you stare at the cloud picture, resting receptors take over, and that’s ehy you see motion in the opposite direction.
This works best with objects with vague or complex outlines. That’s why cloud forms demonstrate this effect dramatically. But trees and rock formations work almost as well, and you could try substituting other pictures.