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Study Notes: Adding Colours

Sunlight (white light) is made up of all the colours of the spectrum: red, orange, yellow, green, blue, and violet mixed together.

We can mix different coloured lights to produce any colour we like.

Only three colours are needed to make light of any colour. They are red, green and blue. These are called the primary colours.

If we shine circles of red and green light onto a screen, the area where the lights overlap is yellow.

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We can mix red and blue light in the same way. The colour of the overlapping area is magenta (which is like purple).

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If we mix green and blue lights we obtain cyan (which is a pale blue or aqua colour).

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Finally, if we mix all three colours (red, green, blue) we get white light.

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The addition rules for the primary colours are:

red + green = yellow
red + blue = magenta
green + blue = cyan
red + green + blue = white

Yellow, magenta and cyan are called the complementary colours. A complementary colour is a colour that combined with a given colour makes white. If, for instance, you combine yellow (ie red + green) and blue you get white light. Therefore:

The complement of blue is yellow (= red + green).
The complement of red is cyan (= green + blue).
The complement of green is magenta (= red + blue).

How is this information useful in spectroscopy? It helps to understand the wavelength that is selected on a spectroscopic instrument for analysis of a certain solution.

For example, a cyan coloured solution means that green and blue light have been transmitted and that red light has therefore been absorbed. The wavelength selected to measure absorption would need to be one that corresponds to red light.

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