1. Shine white light through a prism onto a screen. 2. You should see a clear spectrum of the constituent colours of white light.
Recombination
Recombination can be seen as the opposite of dispersion. The most common form of recombination is taking the three primary colours of light and mixing them to make white light (Fig. 18.11).
Colours
The three primary colours are based on physical wavelengths of light and all interfere to form white light.
The three primary colours are red, green and blue.
FIG. 18.11 The three primary colours mix to make white light
If you mix two primary colours, you will get a secondary colour. These are yellow, magenta and cyan. Projecting the three primary colours shows which secondary colours come from which primary colours:
• Red + Green = Yellow • Red + Blue = Magenta • Blue + Green = Cyan
Complementary colours are primary and secondary colours that combine to give white light.
Combining a secondary colour with the unused primary colour makes white; the secondary/primary pair are known as complementary colours. The pairs of complementary colours are:
• Red + Cyan = White • Blue + Yellow = White • Green + Magenta = White
FIG. 18.12 Mixing colours of light is used for stage shows
To demonstrate the combination of colours
1. Using a ray box and coloured filters, produce the three primary colours independently on a white screen.
2. By mixing each colour, you should notice the secondary colours being produced. 3. Each secondary colour can also be mixed with its complementary colour to produce white light.
LEAVING CERTIFICATE PHYSICS 209
The three colours that combine to make white light are called primary colours.
NOTE
The primary colours listed here are considered the true primary colours based on pure frequency. This is not the case in subjects such as art, where pigment colours are described as being primary.
Combining two primary colours in equal amounts gives a secondary colour.
STS
Mixing colours of light is used for stage effects and in TV technology (Fig. 18.12)
