What it shows
A disc painted with the seven rainbow colours looks almost white when spun fast, because the eye cannot keep up and mixes the colours together. White light is a mix of all colours, which is the reverse of a prism splitting it.
The science
Our eyes hold on to each image for a small fraction of a second, so when colours flash past faster than that, the eye blends them into one. When red, orange, yellow, green, blue, indigo and violet are blended, the brain sees light that is close to white or pale grey, because white light is made of all these colours together. It is not perfect white because paints are never pure and some light is absorbed. The disc was made famous by Newton, who showed that colour is a property of light, not of the glass that splits it.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- An old CD or a circle of white cardboard (free)
- Paints or sketch pens in seven colours, or coloured paper
- A pencil, a marble or a bottle cap as the spinning axle, or a string through two holes
- A ruler and a protractor to divide the circle
- Glue
Safety
For this project: Use a blunt pencil through the disc; an adult should make the holes.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Draw a circle on the cardboard and divide it into seven equal wedges using the protractor, about 51 degrees each.
Colour the wedges in rainbow order: red, orange, yellow, green, blue, indigo, violet.
Push a pencil through the centre so the disc spins like a top, or thread a loop of string through two holes near the centre for a whirling button.
Spin it as fast as you can on a table and look at the colours blur together into a pale greyish white.
Make a second disc with only red and green and see what colour appears when it spins.
Make a third disc with only two colours, blue and yellow, and compare with what paint mixing would give.
Try spinning the seven-colour disc slowly and note the speed at which the colours stop being seen separately.
Write what each disc showed and why the seven-colour disc is not perfectly white.
What you should see
The seven-colour disc turns pale grey-white when spun fast, while two-colour discs blend into a single new shade. Slow spinning still shows the separate colours.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the spinning disc look white?
The eye cannot separate colours that flash past quickly and mixes them, and all the colours mixed together make white light.
2Why is it grey rather than pure white?
Paints are not pure colours and some light is absorbed by the paper.
3What did Newton use the disc to show?
That white light is made of all the colours and that a prism only separates them.
4Does mixing paints give the same result as mixing light?
No, mixing all paints gives a dark muddy colour, because paints absorb light rather than add it.
Take it further
Attach the disc to a small motor or an old toy’s wheel to spin it at a steady speed and test different colour sets.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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