Benham's Disk Illusion

You will need:

Black pen, or a colour printer, plus paper.
1 Nail, pencil, cardboard and glue.

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Instructions

1. Print the picture shown or make your own on paper.

2. Glue the picture on some cardboard and allow to dry.

3. Pierce the middle of the disc with a nail.

4. Insert the pencil through the whole and spin the disc.

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How to teach science, the way primary kids love

5. Do you see any colours on the disk?

6. Slow the disk to 3 - 5 rotations/sec and dim the light.

Benham's disc example that you can make yourself

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Why Does This Happen?

Colour perception is achieved by three types of cells in the retina of the eye called cones.

With the three types of cones, one is more sensitive to red light, one is more sensitive to blue light and the other is more sensitive to green light. Each cone reacts at different speeds to light and keep responding for different lengths of time. You only see white when all three cones respond equally.

The alternating white and black flashes on the disk cause the cones to respond at different times and lengths, leading the brain to believe it is seeing coloured light.