Fami
The disc that makes colours vanish
Colour a disc with the rainbow, spin it and watch the colours turn almost white.
Materials
- 1Disc template, printedor A 10 cm card circle divided into 7 parts
- 7Coloured pencils or felt-tips
- 1Ordinary pencil (for the spindle)or A blunted kebab skewer
Printable template
Disc template · Print it at actual size (100%)
Step by step
- 1
Colour the 7 parts of the disc
Cut it out of the template first. Red to violet, in order
- 2
Push a pencil through the middle
With about 3 cm sticking out underneath, point down
- 3
Spin it like a top
Twist the pencil fast with your fingers and let go
Result
The colours vanish!
Why does it happen?
White light is a mix of all the colours of the rainbow. When the disc spins fast, the colours pass in front of your eye so quickly that it can't see them separately and blends them: the disc looks almost white. With pencils it isn't quite white but light grey, because paints aren't pure light.
For parents
The science, so you can answer
White light is made of all the colours together, and the eye blends anything that changes too fast.
More than 350 years ago, Isaac Newton found that sunlight passing through a prism splits into the colours of the rainbow, and that putting them back together makes white again. The disc goes the other way using a trick of the eye: each image we see stays in the eye for a fraction of a second (persistence of vision). If the seven colours pass the same spot faster than that, the brain doesn't separate them and adds them up. Because pencils aren't pure coloured light (each one swallows some of the light), the sum isn't quite white but a light grey.
What they will ask you
«Where do the colours go?»
They don't go anywhere: they're still painted on. They pass so fast that your eye sees them all at once in the same place and mixes them. When the disc stops, they come back.
«Why isn't it completely white?»
Because paints aren't light: each pencil absorbs some of the light. With real coloured lights, it would come out white.
«Does sunlight have colours inside?»
Yes, all the colours of the rainbow. That's why, when it rains while the sun is out, the drops split the light and a rainbow appears.
«What if it spins slowly?»
You see the colours go round, or a colourful flicker. It has to spin fast so the eye can't keep up.
In real life: TV and phone screens make every colour from tiny red, green and blue dots packed together, and films are still photos shown so fast that the eye sees them as movement.
Animated step by step
Follow it with your phone on the table: it pauses at the end of each step.