Colour without Dye

Peacocks create their stunning colours using tiny shapes hidden on their feathers, not pigments or dyes.

Have you noticed that many birds, butterflies and beetles produce vibrant, dynamic colours? These colours seem to change when seen at different angles. When colour behaves this way, we say it is 'iridescent'. Meanwhile, the colours of human-made things often seem dull and unchanging in comparison. Usually human-made colours use chemicals which can be harmful to the environment.

What if we could make things colourful the way things in nature are? Would you like to live in a world where human-built products change colours in such interesting ways?

For this activity you need a peacock feather or another example of structural colouring.

1. Take the peacock feather (or some other example) and ask pupils to shine a light on it from different angles. Move the feather to view it from different angles. What do they notice?
2. Explain that the pupils will now create this colour effect themselves, to change the colour of light using dispersion rather than chemical dyes.
3. Instruct pupils to cut off the bottom of the PET bottle (a half-litre bottle is sufficient).
4. In a bucket, mix approximately 1/4 litre of water with 3 teaspoons of washing up liquid, 4 teaspoons of (powdered) sugar and 3 teaspoons of oil.
5. Demonstrate how the end of the cut bottle is carefully dipped in the prepared mixture and then slowly blow through the neck of the bottle as large a bubble as possible. Be careful that the bubble stays connected to the bottle.
6. Watch the beautiful rainbow colours appear in the bubble. It is best to do it in the sun, or turn on a light above the bubble, and the iridescent colors of the bubble will then be more pronounced.
7. Invite pupils to blow up their own bubble. It is good to put a washable tablecloth as a mat.
8. Provide pupils with a prism to explore, and describe how light is broken into its individual colours by the prism (see description below).

How come the bubbles display rainbow colours in the light when no colouring has been added? And does the peacock use the similar trick?

Simple explanation 

White light can be diffract into different colours. This can be achieved in various ways. In our case light refracts into iridescent colours when passing through the wall of a bubble. The bubble wall consists of a very fine layer of water coated on both sides with detergent and oil. The thickness of the wall of the bubble affects the colours the light refracts into, that thickness is constantly changing, and therefore the colours of the bubble also change.

It's actually a similar principle to when you look at a rainbow in the sky. Rainbows do not need dye to create colour, only light and something that breaks this light into the individual colours, like raindrops or water vapor.

Specially shaped surfaces can also function similarly to raindrops, changing their colour depending on the direction of light falling on them. You probably know it from the bottom of the CD, but the same principle has long been used by some animals, such as the peacock, whose feathers are normally brown. The micro texture of the feather splits sunlight to create iridescent blue, turquoise and green light.

We humans could take inspiration from these animals and use it in the things we make. Can you think of some ideas where this would be beneficial?

More detailed explanation

Sunlight is made up of light with different wavelengths (called the electromagnetic spectrum). Each of these is reflected at a different angle. By changing the angle of reflection, we can change the colours which are dominant. You can test this out using a glass prism.

Nature uses the same principle called structural coloration. Some animals and plants create micro-structured surfaces which reflect light in the same way a prism works. Depending on the intensity and direction of the light, the colours produced vary. This causes wonderfully variable colours.

The video below explains how a prism works.

The knowledge and use of structural colours by humans can lead to a more visually interesting world, but also potentially has many other benefits. For example, creating colours using structure instead of pigments and dyes could reduce the use of harmful chemicals.

Structural colours are also used on banknotes, which change colour depending on the angle viewed. This helps to prevent counterfeiting (to make sure the money is real).  By more efficiently reflecting light, structural patterning can also reduce heat absorption in buildings and cars. This subsequently prevents them from overheating, so there is no need to use energy-intensive air conditioners as much.

Some companies have used structural colour to make different coloured paints. These paints include tiny crystals whioch form a nanostructure reflecting light to create colour. Clothes manufacturers are using similar ideas to create colourful clothes which do not fade over time.

Some STEAM opportunities include:

• Notice that light is reflected off surfaces and how different surfaces influence how light is reflected.
• Recognise that light appears to travel in straight lines, and explain that objects are seen because they give out or reflect light into the eye.
• Describe how human eyes work to collect light.
• Observe that some materials change state when they are heated or cooled.
• Explore light phenomena such as how light can be split into different colours.

See how the micro-structure of bees create colour (find out more).

Colour-changing security labels inspired by the morpho butterfly (find out more).

Learn how butterflies use light to produce amazing colours (find out more).

Learn more about structural colouration (find out more).