What it shows
Sunlight looks white but is a mixture of colours. Water with a mirror in it acts like a prism, bending each colour by a slightly different amount, so the reflected beam fans out into red, orange, yellow, green, blue, indigo and violet on the wall.
The science
When light passes from air into water it slows down and bends, and when it comes back out it bends again. Each colour in sunlight bends by a slightly different amount, with violet bending the most and red the least, so the colours spread apart; this is called dispersion. The mirror under the water sends the beam back out through the sloping surface at an angle, which makes the spread wide enough to see. Raindrops do the same thing in the sky, which is why a rainbow appears when the Sun shines from behind you onto rain.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A shallow bowl or a tray of water
- A small flat mirror
- A sheet of white paper or a white wall
- Direct sunlight through a window, or a torch in a dark room
- A pencil to trace the colours
Safety
For this project: Never look directly at the Sun or shine the reflected sunbeam into anyone’s 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.
Fill the bowl with water and place it on a windowsill or table where a beam of sunlight falls on it.
Lean the mirror in the water so it tilts upwards, facing the Sun, with part of it under the surface.
Hold the white paper above and in front of the bowl and move it until a patch of coloured light lands on it.
Tilt the mirror slowly until the colours spread into a clear band and name them in order from top to bottom.
Trace the band on the paper with a pencil and colour it in later.
Move the paper further away and see the band grow wider and fainter.
Try with the mirror out of the water and notice that you get only a white patch, which shows the water is doing the splitting.
In the evening, repeat in a dark room with a torch and compare the strength of the colours.
What you should see
A band of rainbow colours on the paper, with red at one edge and violet at the other, that appears only when the mirror is under water and tilted correctly.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What colours make up white sunlight?
Red, orange, yellow, green, blue, indigo and violet.
2Why does the water split the light?
Light bends as it enters and leaves the water, and each colour bends by a different amount.
3Which colour bends the most?
Violet; red bends the least.
4How does a real rainbow form?
Sunlight enters raindrops, is reflected inside them and comes out split into colours.
Take it further
Make a spinning colour disc with the seven colours and show that they blend back into near white when it spins.
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
Easy Science Projects for Class 1 to 5
Simple, safe projects young children can make with a little help
Science projects
Science Projects for Class 6 to 8
Working models matched to middle-school science topics