What it shows
Coloured light mixes differently from paint. Where beams of red, green and blue light overlap, we see new colours, and all three together look almost white.
The science
White light is made of many colours. A coloured sheet lets only its own colour pass. When two coloured beams fall on the same spot, our eyes add them: red and green look yellow, green and blue look cyan, red and blue look pink. All three add to nearly white. This is different from paint, which takes colours away.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Three torches or phone torches (free)
- Red, green and blue cellophane sheets or sweet wrappers (free)
- Rubber bands
- A white wall or a sheet of white paper
- A dark room
Safety
For this project: Never shine a torch into anyone’s eyes. An adult must help set up the phone torches and supervise.
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.
Cover each torch with one colour of cellophane, folded two or three times, and fix it with a rubber band.
Darken the room and point the red torch at the white wall to see a red circle.
Switch on the green torch and slowly overlap its circle with the red one.
Write the new colour you see where they overlap.
Replace the red with blue and overlap with green. Write the new colour.
Overlap red and blue and write the colour.
Switch on all three torches together and see the middle of the overlap.
Draw circles with crayons showing what you saw.
What you should see
Red and green make yellow, green and blue make a bluish green, red and blue make pink and all three together make a pale white.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What does red cellophane do to white light?
It lets only red light pass.
2What colour did red and green lights make?
Yellow.
3Is mixing light the same as mixing paint?
No, light colours add up while paint colours take away.
4Why is a dark room needed?
So the coloured circles are easy to see.
Take it further
Hold your hand in the beams to make a coloured shadow and note the colours of the overlapping shadows.
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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