What it shows
White light is a mixture of the seven spectrum colours. When a disc painted in rainbow sectors spins fast, the eye blends the colours and sees a pale white or grey.
The science
Isaac Newton showed that a prism splits white light into colours and that they can be recombined. On a spinning disc each colour flashes past the eye so quickly that the eye cannot separate them, due to persistence of vision. The brain adds the colours and sees white or light grey. Pigments are never perfect, so the result is a grey rather than pure white.
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 cardboard circle 12 cm wide
- Seven coloured pencils or paints
- A pencil or a marble
- A paper circle to cover the CD
- A glue stick
- A ruler and a protractor
Safety
For this project: An adult must trim the paper with scissors if the CD is to be cut. Keep the spinning disc away from the eyes.
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 paper that fits the CD and divide it into seven equal sectors using a protractor, about 51 degrees each.
Colour the sectors violet, indigo, blue, green, yellow, orange and red in that order.
Glue the paper onto the CD.
Push a sharpened pencil through the centre hole to make a spinner with a tight fit.
Spin it as fast as you can on a table.
Watch the colours blend and note what colour appears.
Spin it slowly and watch the colours separate.
Make another disc with only red and green sectors and see what colour appears.
What you should see
The seven-colour disc looks pale grey or white when spun fast. The two-colour disc shows yellowish blends.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1How many colours are in the spectrum?
Seven: VIBGYOR.
2Why does the disc look white?
The eye blends the fast flashes of colour.
3What is persistence of vision?
The image stays in the eye for about a tenth of a second.
4Why is it not perfect white?
Paints do not reflect colours perfectly.
Take it further
Change the width of the colour sectors and see how the blended colour shifts.
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
Science projects
Physics Working Models Explained
Twenty classic models, how each works and how to make it
Science projects
Best Out of Waste Science Projects
Working models from bottles, cardboard, CDs and old parts