When it rains, and the sun is shining, we may see a rainbow in the sky. It occurs when the sun shines through rain drops, and white light is refracted (bent) as it enters the droplet of water.
On the ground, the rainbow appears as an arc, but if you were flying in an airplane then it can be seen as a circle.
The colours in the rainbow smoothly merge from one to another. They are continuous in their colour ordering. But traditionally we think about a rainbow as having 7 individual colours. This makes it easier to remember the order, and also draw out a simplified version. The seven colours are: Red, Orange, Yellow, Green, Blue, Indigo and Violet. The mnemonic Roy G. Biv is useful as it also highlights the main three primary colours Red, Green, Blue. Values for R,G and B, are often used to code colours on a computer.
When there is only one rainbow (and on the first arc of a double rainbow) red is on the outer part of the arc, with violet on the inside. However when a second arc can be seen, the order of the colours of that second arc appear with red on the inner part of the arc.
Why seven colours?
The summary of the colours of a rainbow, into seven distinct colours, is attributed to Aristotle (384–322 BC) but it may have been developed by Aristotle’s successors Theophrastus or Strato. Newton created a colour circle, to describe how coloured lights can be mixed together. Originally Newton used only five colours, but he added orange and then indigo to match with the seven notes in a major scale, in music. But in reality, simplifying a rainbow to a set of 5 or even 7 steps is rather arbitrary. In addition, Newton named his colours different to how we would now name them. What he called “Indigo” we would now consider to be “Blue”, and what he called “Blue”, we would say it was “Cyan”.
Mixing paints versus mixing lights
There is a difference between mixing lights, and mixing paint. Additive colour mixing, is the term used when mixing lights, whereas the phrase subtractive mixing is used when mixing paints. Probably students learn more about mixing paint, in comparison to learning how to mix light! For example, I’m sure you know that when yellow paint is mixed with blue we get green paint. But mixing lights is different. And on a computer we need to think in terms of mixing light. We deal with subtractive colour mixing on a computer.
You have probably described a car to be deep red, or perhaps light red. Other reds may be described as being bright red, or another colour may appear as a subtle red, or perhaps a pink. We are describing the colour hue. In each of our examples, our primary (underpinning) colour is red.
In language we express different colours by adding different adjectives. Light blue, dark blue, pastel blue, vivid blue, etc.
We can change the lightness of the hue, by mixing in more black, or more white. This same concept is also known as value, or tone.
- To make a lighter hue you need to mix in more white. This would reduce the darkness of the colour. This is also called a high-value colour.
- To make a darker hue you need to mix in more black. This is also called a low-value colour.
Finally, in graphics and imaging, it is common to talk about the saturation of a colour. We can think of saturation as the brightness or intensity of the colours in a scene.
To create the rainbow spinner you will need:
- Plain cardboard (or white solid card)
- Coloured pens (one for each colour of the rainbow!)
- Compass, or something circular such as a plate
- Optional protractor, to calculate the angles
Follow these instructions to make your rainbow spinner.
- Set the compass to 5cm radius. Or get a plate about 10cm diameter. Draw a circle on the cardboard, and cut the cardboard out, to get a circular disc.
- Divide the circle into 7 equal parts. Mark the seven segments using a pencil to give you a guide of what you will colour in. If you have a protractor then you can use it to mark out the segments, it would be about 51° each segment (ie., 360/7=51.4).
- Colour each segment with different colours, one for each colour of the rainbow.
- Make two small holes (about 1 cm apart) that are either side of the centre of the disc. Thread the string through these holes, such that they create one large loop with the spinner in the centre.
To run the spinner: hold an end of the string in each hand. Slowly turn your hands in a circular motion (like you were holding two whisks, one in each hand). This will start to spin the disc and the string will become twisted in the same direction. Once the string is twisted, then gently pull the string with both hands. It should start to turn. It should speed up and then. Release the pressure and it will start to slow down, now is the time to pull again. Much like you were planing an accordion.
Now you know more about colours and rainbows.