Mixed-Light Makes White, but Mixed-Paint Makes Black?
We are told that white light is made up of the colours of the rainbow, but why does mixing paint colours create black? Surely this must be a glitch in the matrix where nothing we see is real! Well.. it turns out this is sort of true. When light shines on paint, the paint acts as a filter: it will take away certain colours of the light. This doesn’t just apply to paint though, in fact, you are seeing every single object in your room through filters! Maybe seeing is not so black and white after all…
We are living in a world where filters add and subtract light colours. Before reaching your eyes, the lights shining on mixed paint has to go through more filtering compared to mixed light. To understand this, let’s take a look at how we see colour, how colours are added and finally, how they are subtracted.
How do we see colour
Our ability to see colour comes from visible-light waves which reflect off our surroundings into our eyes. However, not all light waves are created equal and each colour has its own signature wavelength. For example, red light waves are longer while blue light waves are shorter.
Wavelength differences are the key to decoding the colours we perceive. Our eyes are home to around 6-7 million photoreceptor cells called ‘cones’ which respond to red, green and blue light. Using cones, information about the types of colour lightwaves is sent through the optic nerve into the brain’s visual cortex for processing. When wavelengths from all colours are detected, we see white. When no light is detected, we see black.
Additive colour-mixing with light
Let’s recall how white light is composed from colours of the rainbow. Additive colour mixing happens when different colours of light are added when they reach your eye. As more colours are added to the mix, the colours we see get lighter until eventually they reach…white. In fact, the colours on computer screens is a result of additive colour mixing! Your screen includes tiny rows of red (R), green (G) and blue (B) lights at varying levels of brightnesses. Try standing back from your screen: you may notice that the brighter sections of the picture (above) blur into a lighter colour whereas the dimmer sections form a darker blur.
Subtractive colour-mixing with paint
Returning to the idea of colour filters, objects absorb certain wavelengths of light and reflect others. For example, red paint absorbs all other coloured light except for the red wavelength – just like a filter. Therefore, when you look at red paint, you are seeing the reflected red light. So what happens when you mix different coloured paints? As you add more paint colours, more coloured light is absorbed into the mixture of filters. Eventually, hardly any light will be reflected resulting in an almost-black colour. This is called subtractive colour mixing because of how the paint subtracts colours from the light source. So, next time you decide to mix a colour, what filters will you be choosing?
To find out more, check out these links below:
Watch a coloured disk change into white was it spins: https://www.youtube.com/watch?v=7iV1m4j2wJQ
Light mixing: https://www.youtube.com/watch?v=ksS-FJ3B8Og
More Colour Science with the Pinkest Pink and the Blackest Black: https://www.newscientist.com/article/2156776-painter-painter-of-the-wall-whats-the-fairest-colour-of-all/