Colour Answers

by Deb on December 2, 2011

Colour Answers

1. We see because our eyes pick up light that has bounced off the things around us. It feels a little backwards, but the colour something appears is the colour it reflects rather than absorbs. A green book would absorb every colour except green and bounce the green back to us.

Colour is determined by the wavelengths of the light energy bouncing around and how our brain interprets it. There are three types of cone cells in your retina that are most sensitive to different wavelengths. If the light hitting your retina activates mostly S(hort) cones, your brain will interpret it as dark blue. It is the mix and proportion of the cone cells that are excited that makes up the full spectrum of colours we perceive.

subtractive colour2. Mixing light and paint are quite different, because one is additive and one is subtractive. We are used to paints and pigments, that is subtractive. As above, when we see a colour pigment it is the light that is being reflected to us. Starting with white or mixed light, some of it is absorbed by the pigment and only a fraction is reflected. When you add another pigment it will absorb more and reflect less, subtracting wavelengths from what is reaching our eyes. Eventually when there are many pigments all the wavelengths are being absorbed and none reflected, so we see black.

Additive Colour MixingMixing coloured light is different because you are starting with limited wavelengths and then adding more. If you shine a red light onto a screen, only red will bounce back to you and it will appear red. However if you shine a green light as well, your eyes are getting both long waves from the red and medium waves from the green. Your brain interprets this mix of wavelengths as yellow. Adding more lights adds to the wavelengths and cone cells being stimulated, when you add them all you eventually get white light.

The video at the bottom demonstrates this and has a fun activity as well.

3. While colours obviously exist, they fall along several continuous spectra. I say several spectra, because the hue, what we label as the actual colour, is only one part of it. There is also tone which describes how dark and light the colour is and intensity for the strength. Then you can get really technical with things like opacity, brilliance and texture.

The way we divide these spectra, where we put the lines and how we describe things, are purely cultural. English speakers focus on hue and tend to divide the spectrum into six colours – or is it seven? It was known that water or a prism could split white light into a spectrum when Isaac Newton experimented with it and wrote Opticks in the 17th century. He divided the spectrum into 7 colours, but this was based on a philosophical belief rather than anything physical – he believed there was a connection between musical notes, the days of the week, the known objects in the solar system and colours, so he needed seven of them. (Thanks to CatWay for putting me on to this.)

Many people find it difficult to distinguish indigo from blue or violet, and in spite of the ROY G BIV mnemonic most of us have learnt we act as if there are six main colours, especially when we are using things like primary and secondary colours. But then there are brown, grey and black and white as well! That’s even more.

Other cultures name and divide the spectrum in different places. In Japan and China blue and green are considered shades of the same colour. There can also be cultural elements such as distinguishing natural and artificial colours or it can depend on what is coloured, such as calling a horse chestnut instead of orange (bright brown?). There has been work done linguistically on colour description that shows languages go through basic stages in how colours are named. The most basic is a division into ‘dark’ and ‘bright’ and colours are added in a set order until there are at least 11 agreed upon basic colours.

4. I’ve often wondered about blue eyes, because I know humans don’t have a blue pigment. Most of our colours of hair and skin are made of eumelanin, or black, and phaeomelanin, or red. It’s a little more complicated than we were taught in school, there are several different genes involved, but making blue itself is a lot simpler than I thought.

Iris close-up

It turns out that the melanin that gives our eyes colour is on a thin layer of cells at the back of the iris. In brown eyes there is a lot of melanin and it can be seen through the bundles of tissue in front of it. In blue eyes there is only a little bit of melanin. The tissue in front, which is all those fibres you can see, scatters light in the same way dust and gases in the atmosphere do. And just like the sky, the light that is scattered most is blue, so that’s what you see.

5. As the caption says the wings at the top are wasp wings, but absolutely nothing special has been done to them. Except putting them on a black background. It turns out wasps and flies have wings every bit as colourful as butterflies, and we never noticed because we put them against white backgrounds.

fly_rainbow

They were actually discovered in the 1800s but basically ignored since then. However Ekaterina Shevtsova has now studied them and discovered that they are not random but unique to each species. They are caused by ‘thin film interference,’ like the rainbow patterns of oil on water. Most light gets through, but some is reflected from the top of the wing and some from the back. The two slightly different reflections interact to make a colour, which depends on how thick the wing is and how far apart they are.

On a white background they are overwhelmed by all the light reflecting from it, but on a black (or green!) background they can be seen. In fact, now that I know they’re there and to look for them I’ve seen them myself when flies are sitting on dark furniture. They’re fun to look out for. As well as helping to identify different species, they could be involved in all sorts of insect puzzles, from fly courtship behaviour to vein patterns.

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{ 2 comments… read them below or add one }

Leonor Miller December 3, 2011 at 6:14 pm

Heard about mixing lights for the first time. Please tell me how he mixed the lights in “liquid” form? (in the video)
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Deb December 3, 2011 at 8:19 pm

He’s cut open some glow sticks and used the glowing liquid inside. Mixing them acts the same way as mixing lights. Or you can do it the old-fashioned way and put cellophane over torches.

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