We are able to see colours because our eyes can distinguish the different wavelengths of visible light from the electromagnetic spectrum, as different colours. The longest wavelength that we are able to see is red, whilst the shortest is violet. The other colours of the spectrum lie between these two. The full spectrum is:
- Red
- Orange
- Yellow
- Green
- Blue
- Indigo
- Violet
The mnemonic, Richard Of York Gained Battle In Vain, is just one of many used to help remember the correct order of the spectrum. These colours are all contained in white light. When white light is passed through a prism it is split into the seven constituent colours.
Of the seven colours, three – red, green and blue – are primary colours and can be combined to create all the other colours in the spectrum, and indeed almost any other colour. Where two primary colours overlap, they produce secondary colours.
Primary Colours | overlap to make | Secondary Colours |
|---|---|---|
Red and Blue |
> |
Magenta |
Read and Green |
> |
Yellow |
Green and Blue |
> |
Cyan |
Red, Green and Blue |
> |
White |
The nerve cells on our eyes are only able to detect these three primary colours, so we see other colours because of the very many combinations and proportions used. We see the colours of objects because when light is reflected from an object into our eyes, the only colours to be reflected are those that match the pigment in the colour of the object. The rest is absorbed.
For example, if you look at a red object under white light, only the red light is reflected (the other colours are be absorbed). Similarly, if you look at a red object under a red light, the object would still appear red since only the red light is reflected anyway. If however you were to look at a red object under a blue light (through a filter which only allowed blue light to pass through and blocked red and green), then the object would appear black since the pigment in the object would absorb the blue light and there is no red to be reflected. So shining different coloured lights on objects can cause them to seemingly take on a different colour. A cyan object, for example, looks black in red lighting but green in yellow lighting. To work out what colour different coloured objects will appear, start with the primary colours in the light, take away the colours that the object absorbs and you are left with the reflected colours. (Note that is very difficult to obtain good, effective colour filters in primary schools and virtually impossible to set up conditions to mix coloured lights to obtain satisfactory secondary light colours or white light.)
The primary colours of light and the primary colours of paint and pigments are different. Whereas the colour mixing of light involves adding colours, colour mixing in paint involves subtracting colours. Pigments absorb different colours of light (i.e. blue light absorbs all colours except the blue range, red absorbs all but the red range etc.). If pigments were ‘pure’, mixing all of them would produce black, as all colours would be absorbed. In reality, paint pigments are mixtures and rarely absorb all the wavelengths. Thus mixed paints always reflect some traces of colour so, when mixing them together, we are more likely to see a muddy brown. The primary colours of pigment are related to the secondary colours of light but we usually refer to them as yellow, blue and red.
If the proportions of pigment and light are varied, then the number of possible colours produced is huge. Even though we may sometimes find it difficult to distinguish different colours, our eyes are actually capable of distinguishing more than 20,000 shades.