• Question: Why are red, green and blue the primary colours of light?

    Asked by imctaggart to Adam, Catherine, Karen, Leila, Nazim on 15 Mar 2012.
    • Photo: Adam Stevens

      Adam Stevens answered on 15 Mar 2012:

      BRILLIANT QUESTION.

      So this is really hard to answer, but first you need to know that there’s two totally separate parts to colour. There’s the biological and psychological interpretation of light, and there’s the physical reality.

      So hold onto your hat, but I’m about to blow your mind – There are actually two types of yellow.

      BOOM.

      There’s the yellow that’s made from Red and Green light mixed together. But there’s also just plain yellow light that has always been yellow.

      We see them as the same, but if you get a computer and a light sensor, it sees the first one as Red+Green, not yellow. The second one it sees as yellow.

      This means that colour mixing is a psychological thing.

      We can actually trace the three primary colours back to the detectors in our eyes. We have three types, one for red, one for green and one for blue and anything else is mixed in between the two.

      Other animals have different primary colours (or none at all) and some have more than 3 types of receptors so can see further (ultraviolet light for example).

    • Photo: Karen Masters

      Karen Masters answered on 18 Mar 2012:

      Great answer by Adam.

      The only interesting thing I wanted to add is that I heard some of what we think about colour is cultural too. Like the different shades of blue and green – you might think it’s obvious which are blue and which are green – but I read that there’s a tribe in Africa who can’t tell the difference, but have two names for brown (I think it was brown) and reliably identify different shades of brown we cannot distinguish as one or the other. I thought that was really interesting.

    • Photo: Nazim Bharmal

      Nazim Bharmal answered on 18 Mar 2012:

      If you look up really close to a TV screen, you can see three colours of “bulb” (or LEDs if you have a more modern TV than me!). They are red, green (sometimes 2) and blue. By putting out different brightnesses of red, green, and blue, you can make the eye see different colours.

      That is because like Adam said, our eyes are sensitive to roughly three colours: no surprise that these are red, green and blue! (Actually, some women are sensitive to 4 basic colours: red, blue, and 2 types of green so they can see more colours than most people!)

      When our eyes see some green and blue, we call that mixture ‘purple’. When real purple light comes into our eyes, we detect equal amounts of green and blue and so we see purple! That means a TV “fools” our eyes into seeing lots of different colours by using only 3. (We also fool the eyes when we watch 3D films, although not completely so that is why you can get a headache after a while of watching 3D.)

    • Photo: Leila Battison

      Leila Battison answered on 19 Mar 2012:

      Awesome question!

      Basically it comes down to how we define different colours, which isn’t as easy as you think. Each colour has a particular wavelength of light, but they all grade into eachother, and you can’t really draw a line between different colours…

      But in our eyes we have special cells called ‘cones’ that are designed to pick up very specific wavelengths of light, at blue, red, and green. So our brain is most receptive to ‘pure’ versions of these colours, so we define them as primary colours.

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