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Dear Professor Science, When I mix red and green paint I get brown, but when I mix red and green light I get yellow! How does this happen? It's all down to whether we are adding or subtracting colour, and how our eyes and brain perceive it.

White light is made up of lots of different wavelengths, if you split up the white light, with a prism or drop of water, you can see them as a spectrum of all the colours of the rainbow.

When light hits a coloured object the colour that we see depends on which wavelengths of light are reflected back to our eyes. The colours that are not reflected back are absorbed by the object.

If you shine white light onto red paint it absorbs all the wavelengths except the long wavelengths we see as red.

Green paint absorbs all the wavelengths except those middle wavelengths which we perceive as green. When they mix we see brown, the colour we see is what's left after other wavelengths have been taken away so this is known as subtractive colour mixing.

A coloured light is made up of a particular wavelength, by mixing coloured lights we are actually adding different wavelengths together, leading to our brains perceiving a new colour, this is known as additive colour mixing.

The different wavelengths of light activate the cone cells at the back of the eye which pass the message to the brain. There are three different kinds of cone cells, some that are most sensitive to red-ish light, some that are most sensitive green-ish light and some that are most sensitive blue-ish light. However we can see a much greater range of colours as when the cones send messages at the same time the brain interprets the mixture as a new colour.

A mixture of the wavelengths of red and green light add together to produce a wavelength we see as yellow.

There's a new laser light and shadow show called In the Dark in the Science Theatre currently running at The Life Science Centre which explores how light travels in straight lines and how polarising sunglasses create weird and wonderful effects. (Warning: strobe lighting) This column is kindly donated by the scientists at the Centre for Life in Newcastle. If you have a question for them, please send it to Alternatively, to find out more about what's happening at Life, log on to
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Publication:Evening Chronicle (Newcastle, England)
Date:Oct 11, 2012
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