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Why are the primary colours in art different to the primary colours in Science?

Why are the primary colours in art different to the primary colours in Science?
Rebecca Charlton from Cumbria (Age: 5-14)

Why do we have primary colours?
Emilie from West Midlands (age 5-14)


One Response

  1. Hi Rebecca,

    I suspect you are thinking of the paints you need to mix to get other colours, compared to the lights you are told in a science lesson must be mixed to get other colours (or white). Respectively, these are referred to as subtractive and additive mixing, as I will explain.

    Our eyes have three types of receptor that are most sensitive to light in three different colour ranges – roughly red, green and blue. Different coloured lights have different wavelengths (like tuning to different radio or TV stations). Yellow, for example, has a length intermediate between red and green. If we look at a yellow light it will to some extent activate both the red and the green receptors. With them both active our brain interprets this as ‘yellowness’. Obviously they would also both be active if we shone red and green light simultaneously – the mixture would look like yellow and our brain wouldn’t be able to tell the difference between ‘the real thing’ and this mixture. For this reason red and green are called primary colours – light of those wavelengths can be mixed in various proportions to make other colours like orange and yellow. Blue is the remaining primary – add that and you can make cyan, and if you put all three together it looks white. This process is additive – the coloured lights are added together.

    An artist’s paints do not shine with light. Instead, when white light (a mix of all the wavelengths) falls on them they absorb a great deal. Just a little is reflected and the wavelength determines what the colour looks like to us. For example, red paint looks red because the other colours of shorter wavelength, all the way down to blue, are all absorbed. Now think of green paint: that absorbs red, but reflects green. If it absorbs red, then mixing it with red will tend to stop the red light being reflected and the mixture will be a dirty brown. Thus paints (pigments as they are called) take away colour from white, and each extra pigment takes away some more – subtractive mixing.

    Typical pigments are those used in computer printers – cyan (that reflects blue and green), magenta (that reflects blue and red) and yellow (that reflects red and green). Because each reflects two colours, when they are mixed the common colour is reflected. E.g. yellow and cyan will look green, because both reflect that (there’s only one each of the red and blue so they get absorbed by the other pigment). Thus these printer pigments serve as primary colours, but whereas additive mixing of all primaries gives white, when primary pigments are mixed every colour is absorbed and they look black.

    Hope that helps.

    Best wishes, Peter

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