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Primary colours
Primary colours
The three colours from which all other colours can be made. They are red, green and blue. That is why, for example, the glowing dots on the inside of a television screen are designed to give out red, green and blue light.
When light is sent directly
to the eye, the primary colours appear to add together. Red and green make yellow. Green and blue produce cyan. Blue and red make magenta. Red, green and blue produce white.
However, when light reflected from the world around us reaches the eye, the primary colours are different. The primaries become magenta (bluish-red), yellow and cyan (blue–green).
These reflected primary colours absorb some of the white light that reaches them and filter out,
or subtract, certain light waves. These primaries are the opposite (called complementary) colours from the red, green and blue primaries. For example, a magenta filter will take out the green part
of the light, while a yellow filter takes out the blue and cyan takes out the red. If you placed a magenta filter over a yellow filter and then added a cyan filter, or if you added magenta to yellow or cyan dyes, no light would leave at all. That is how the black colour of paint is created. (See also: Colour mixing.)
Prism
A wedge-shaped piece of
glass or plastic. Prisms
have played an important
part in the study of light.
They were used by Sir Isaac Newton to separate white light into a rainbow of colours (see: Primary colours). Using a prism, he was able to show that white light was not a single colour at all, but a mixture of all other colours.
Prisms are still used in science to make rainbows, or spectrums, of coloured light beams.
Glass prism
Orange
Narrow beam of white light
Green
Yellow
Cyan
White
Red
Indigo
Magenta
Blue–violet
 Primary colours – The primary colours are red, green, and blue. If they are placed on a spinning
top, as shown left, the colours will appear to merge together to make white.
The primary colours all have complementary colours. They are shown in a diagram first devised by Sir Isaac Newton.
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