• Question: Why isn't brown the colour of the rainbow?

    Asked by azzle to Anna, Hayley, Iain, Rebecca on 24 Jun 2015. This question was also asked by nihou1234, BOB.
    • Photo: Iain Bethune

      Iain Bethune answered on 24 Jun 2015:

      A rainbow contains all the ‘colour’s that our brain & eyes can perceive in the electromagnetic spectrum. Each colour is formed by light of a particular wavelength – actually the rainbow does not form into 7 convenient bands, they are blurred because between e.g. green and blue there are some greenish-blue colours.

      There are three ‘primary’ colours of light – Red, Green and Blue, that are detected by different sensory cells in the back of our eyes (the Retina), and all colours that we see are in fact different levels of response of these cells to light in varying amounts. Brown is a mixture of mostly Red with a smaller amount of Green and Blue mixed in. Nowhere in the rainbow do you see Red and Blue light at the same time – there is Red+Green (yellow) and Green+Blue (cyan) – but no Brown!

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