Science you can see: rainbows and colours

Rainbows appear in the sky after it has rained, when the sun shines on tiny drops of water in the sky. Rainbows form an arc (or a 'bow') of colours in the sky, with red light on the outside of the arc, and violet inside.

The order of the colours is always red, orange, yellow, green, blue, indigo (a very dark blue), and violet. This order is remembered by the mnemonic (or memory aid) Roy G. Biv or alternatively VIBGYOR. The range of colours is called the 'spectrum'.

Rainbow colours result from the splitting of the white light of the sun, or even the white light from a light bulb. This light looks white but is actually made up of a blend of all the colours of the rainbow. Each colour has a different energy - with violet having the most energy and red, the least.

If light from the sun is white, why is the sky blue? Normally the sky appears blue, because the higher energy blue part of the spectrum in the white light from the sun is scattered around more in the gases of the sky. At the horizon, less scattered light reaches your eyes, so the sky usually seems paler. Sometimes at sunset, particles of dust scatter the light differently and red/orange colours may predominate.

Natural phenomena such as rainbows have always fascinated humans. In Aboriginal culture, the Rainbow Serpent (Nagalyod) is one of the Dreamtime creators who moves around the sky. The Rainbow Serpent is seen as both a creative spirit and a natural destructive force and is always associated with creeks and billabongs. It is a source of life and protection for the land.

Making rainbows

  • Rainbows, or the spectrum of colours can be seen when there is a spray of water in sunlight e.g. rainbows may form in the spray from waves at the beach, as waves break on a sunny day. These are called spray bows.
  • You can make a rainbow outside in the sun, with a spray of mist from a garden hose, or water from a spray bottle. Often near waterfalls or fountains, the spray throws up a rainbow.
  • Rainbows can also be seen if you darken the room, and shine a torch at an angle to a magnifying glass, and bounce the reflection off a pale wall.
  • A nice way to make rainbows is in bubbles blown from soapy water.
  • When you look carefully, you may see rainbow colours reflected from a glass of water left in a sunny spot.
  • Sometimes you will notice a rainbow if petrol from a car has spilt on the road, and the light is reflected as many colours.
  • Flip over a CD or DVD and see that the fine lines over the back of the disc make light split into the rainbow spectrum.
Making rainbows Image - photo of a CD reflecting light

More fun with colours

Coloured cellophane produces magic results since colours become muddled up when placed under it.

  • Try drawing in red colours on a white sheet of paper, then put the paper under red cellophane and watch the drawing disappear into the now pink page!
  • Highlighter pens work best for seeing the magic under cellophane. Draw a smiley face with a blue highlighter, and then draw a sad face in pink over the top. Which face do you see under the red cellophane? Under the blue? Do your own experiment and try both cellophane colours together.
  • Try drawing a green spider on a yellow web, and put this picture under yellow, then green cellophane. What changes do you see?
  • Put sheets of different colour paper under the cellophane and see what colours you make. Our eyes are easily fooled.

Trick your eyes with spinning colours

Colours can appear to mix up when they move past your eye quickly.

  • Trace around a 20-cent coin on a piece of cardboard. Then carefully cut out the circle. Divide it up into quarters using a biro and a ruler (just like you would cut up a pizza!) and colour alternate quarters in red and blue biro. Mark the centre of the circle and put a very small pinhole in it. Now using a toothpick, make a top, by putting the stick through the hole. Spin your top on the table. (It might take a bit of practice to spin it successfully.) You will see the colours blend to appear purple while the top is spinning.
  • Try drawing black spirals on your circle. Try putting a small square of different coloured cardboard over the circle on your top as well.

Lolly rainbow

  • The rainbow colour trick is most fun when you take a big white bowl, with a flat bottom, and put a line of lollies across the middle. (Smarties, M&M's or Home Brand Choc Beans all work.) Use a lolly of each colour in the rainbow - remember Roy G Biv! Carefully pour a thin layer of cold water into the bowl, and leave it sit very still for a few minutes. The coloured dyes dissolve off the lollies, forming a bright rainbow.

Sugar cube variation

  • A very pretty effect can be achieved by putting a single sugar cube at the centre of a circle of six coloured lollies in the bottom of the bowl. After adding water, watch closely and you will see that the sugar repels the dyes, and at one instant a definite star shape forms around the cube, as the colours flow outwards. Experiment with moving the lollies closer, or putting four or eight lollies in the circle. More fun than eating the lollies!
Sugar Cube Variation Image - photo of smarties and a sugar cube in a bowl

Cabbage juice colour changes

For the really adventurous, boil up a quarter of sliced red cabbage in water and leave it to cool. Pour off the purple juice. Taking small samples of juice in a spoon or glass, add a little sample of a variety of household products and watch the colour change. Try adding salt as your first test. There is no colour change. This is your scientific control.

  • Acid liquids like lemon juice or vinegar will make the cabbage juice go pink.
  • Basic materials like bicarb of soda (baking powder), or washing powder turn the purple juice to blue.

This juice is an example of an acid/base indicator - much like the indicator you might use to check the pH (or acidity) of your swimming pool, to keep it near neutral.

You could try this experiment using juice from a can of beetroot - rather than making the cabbage indicator. Beetroot juice is already acid, so lemon juice won't make much difference to the very dark red colour. Strong bases like laundry products will change the coloured dye to purple or even blue.

Vegetable dyes have been used for centuries to colour paints and materials for coloured clothes, to decorate and brighten our human activities.

Important to note

All activities are designed to be performed under the supervision of a responsible adult who is aware of any conditions which might preclude the safe participation of the child with regard to allergies to food, or other household or environmental agents.

 

For more information about rainbows and colours:

http://www.weatherwizkids.com/rainbow2.htm (examining primary colours)

http://www.youtube.com/watch?v=6OWxI1nD8Hk (make an origami rainbow)

http://en.wikipedia.org/wiki/Rainbow (about rainbows)

Also check the websites of the Bureau of Meteorology, Museum Victoria or the Powerhouse Museum Sydney.

Article written by Kathy Andrewartha (B.App.Sci., M.Sc., B.A., Grad Dip. Info. Sci.). Kathy studied Biochemistry at Swinburne and La Trobe Universities and has worked on research topics involving both plants and animals. She enjoys teaching science at all levels. She particularly likes presenting science to young children, and is passionate about stimulating active learning and analytical thinking about scientific topics.