What’s your favourite colour?

What’s your favourite colour? Anyone who has socialised with small children will have been confronted with this serious-faced interrogation at some point. It’s the sort of question that erupts as soon as young kids learn to verbalise the jumble of perceptions filling up minds that are untidier than bedrooms. It’s the sort of question that is too often discarded in the mis-named process of growing up.

Colour wheel

So what do you answer? Blue? Green? Girls aren’t encouraged to say pink these days, though the same answer from boys may be applauded.

Do you try to be smart and say something like ochre? Or puce? Clever. You get to show off and educate the child at the same time.

But if you really want to twist their brains until you hear that bubble-wrap popping noise, you’ll use the answer that I would give: X-ray.

X-ray is a colour. It is. We can’t see it because our eyes are only tuned to visible light. But X-ray is definitely a colour. Red light is an electromagnetic wave with a wavelength — the distance between successive crests — of about 700 nanometers (nm). That’s just under one thousandth of a millimetre. And the only difference between red light and blue light is that the wavelength of blue light is smaller, at only 400 nm. The wavelength of an X-ray is over a thousand times smaller still, so small it is hard to imagine. It’s only about one ten millionth of a millimetre (0.1 nm).

X-ray is my favourite colour because it allows us to see inside things. We can’t say what colour X-rays look like — we don’t have the experience or the words — but because they will darken photographic film (or, these days, a CCD chip), we can still use them to make images. And since they penetrate matter and objects rather than bouncing off the surface like regular colours, those images can be literally extraordinary.

If you have ever been lucky enough to break a bone, you will have had a picture taken with X-rays that reveals the details of the injury. The X-rays go right through you but are absorbed differently by muscle and bone and so cast spooky but informative shadows. The photographer Nick Veasey uses X-rays to take pictures of the insides of lots of different things. Here’s one he took of a plane. Yes, a plane

From Nick Veasey's Plane

X-ray of a plane by Nick Veasey

The other great thing about X-rays is that you can use them to see atoms and molecules. Since the X-ray wavelength is so small, the size of a typical atom in fact, it’s penetrating power can reveal the unseen structure of matter. You have to trick that matter into forming crystals (a story for another day) but anything that can be induced to crystallise — and that includes salt, proteins and even viruses — will yield up its inner secrets. With the colour X-ray you can paint a picture of a landscape that few people ever see.

Human Serum Albumin with bound fatty acids

X-ray structure of albumin, which carries fat molecules around the body

Which is a great shame, because all of the atomic interactions that make the world and all of the interactions between molecules that make you you happen in this unfamiliar territory. If more people — they don’t have to be children — knew even a little bit more about what happens in the universe of the molecule, they would ask all sorts of interesting questions.

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54 Responses to What’s your favourite colour?

  1. I don’t think X-ray is a colour! Colour is a quantifier of the visible segment of the electro-magnetic spectrum. X-rays lay outside of this segment and are therefore not a colour. Am I a pedant? Yes! Nice post though ;))

    • Stephen says:

      You are being a pedant, philosophically so, but you do have a point. Colour is a subjective perception.

      But by my lights, and following the insight from physics that different colours correspond to different wavelengths (notwithstanding the contribution of the differential sensitivities of the cone cells in our retinas), I think I am right too.

      At the very least, the tactic of calling X-ray a colour should still lead to interesting conversations with young children.

      • rpg says:

        Well, seeing as ‘colour’ is part of our own definition of what we perceive, then by definition something that we can’t perceive doesn’t have colour. I’m with Thony.

        You might ask what colour is a protein molecule. You could say ‘colourless’ and we’d understand you—but by your argument you could also say … well, what *is* a couple of nanometres?

        Yes, what we call colours is but part of what we call the EM, or light, spectrum, but light isn’t what we see, it’s just what enables us to see, no?

        It’s certainly an interesting way to get a child hooked. Then they will learn, and turn round and say why you’re wrong 🙂 — at least until we all decide to redefine ‘colour’ so as to include the wavelengths we can’t perceive with our senses.

        • rpg says:

          Put another way, if ‘X-ray’ is a colour, where is it on, or how do you mix it from, that colour wheel you posted above? What, in effect, is its RGB (or CMYK, come to that)?

        • Stephen says:

          Correct, technically – by our perceptual definitions. But I think you can see the method in my madness. There is an argument by extension here that makes a kind of sense.

          Most proteins are colourless (haeoblogin being a notable exception), in having no visible colour. But since they absorb X-rays, they would have a ‘colour’ in that end of the spectrum, if only we had a biological means to perceive it. Which begs the question: if we augment our perception by artificial means (an X-ray spectrometer), can we not talk in terms of colour?

          • rpg says:

            Proteins don’t absorb X-rays though. Crystals diffract them.

            I like the idea of haemoblogging.

            It’s an interesting idea. And I think you’re right in the sense that 2+2=5 if 2 is large and 5 is small. *Technically*, though, 2+2=4.

            • Stephen says:

              haeoblogin? Oh dear, that’ll teach me to comment late at night by iPhone.

              Protein crystals do absorb – though it may be mostly due to scattering effects; there is a wavelength- dependence to this (absorption increases with the 4th power of the wavelength) – I think it’s the same phenomenon that gives the sky it’s blue colour.

              But it’s true there isn’t much absorption-edge type absorption going on. For that you need to add selenium or another heavy atom.

          • Grant says:

            You could always say what’s your favourite frequency or frequency range.

            The quibble or philosophical debate about ‘colour’ aside, I like the post. My Ph.D. supervisor once said to me (a long time ago, when I was a student) that one thing he wished he’d done was to solve an x-ray structure. I feel that way myself at times.

  2. This post makes me wonder if certain people can see farther into the red or blue ends of the spectrum than others, similar to how some people can hear much higher frequencies. I wonder.

    Fun fact: my mother’s first job in Canada was as a lab technician performing X-ray microscopy. An interesting technique that unfortunately went the way of the dodo rather rapidly. Must ask her to dig out an old micrograph or two if she still has some kicking around.

    • Almost certainly yes.

    • Laurence Cox says:

      Certainly yes. People who have had their eye lens replaced because of a cataract can see slightly into the ultraviolet. Everyone can see into the infrared (that is beyond 700 nm) but the sensitivity of the cones falls off faster than the sensitivity of the rods, so there is no sensation of a change of colour with increasing wavelength. By 1060 nm, the sensitivity of the rods is so low that the detection level is comparable with the damage level for Nd:YAG lasers.

      • I think (but don’t know) that variation in colour perception exists within the visual range, too. I often see “more green” than other people – e.g. I’ll consistently see what others see as a grey shirt as being green-grey, or what they see as being a blue sweater as being closer to teal.

        My Mum says it’s because I have green eyes! 😀

      • Stephen says:

        I don’t have green eyes but do have some grey-green difficulties. I have a coat I bought as grey that my kids insist is green. I can no longer trust myself to go shopping for clothes…

        Of course, my kids could just be messing with my head.

        • Steve J says:

          Some GFP tagged protein will set you straight for sure 🙂

          • Points of contention at my home usually involve red/brown and yellow/orange. Green/blue too, sometimes.

            Case in point – the pair of scissors in the hall table, which I swear has brown handles. My wife claims red. Those colours that are inbetween yellow/orange and green/blue are tricky, too. But I don’t think this is a biological phenomenon, just a psychological one as to what I “think” is green vs. cyan vs. blue vs. whatever. I have no sensible way to test this hypothesis, of course.

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  4. Liz says:

    Now try to imagine you’re 4 years old and have no concept of the EM spectrum. This revelation, that there are “colours” that humans can’t see with our eyes but can detect in other ways, would fundamentally challenge how you see the world.

    I can see how use of the word colour is contentious, but the concept of wavelength is a bit abstract for your average 4 year old.

    This is a simple way to convey the awe & wonder of science and I love it.

  5. jaclong says:

    But wow if we *could* see X-Rays and see some more internal structure of the world. Things would look a little like The Matrix. And just imagine how tidy you’d have to keep your cupboards – just shoving stuff behind a door wouldn’t mean people thought your house was tidy.

  6. What about infra-red and ultra-violet? They sound more like colours, and the lamps that produce them encroach onto the visible spectrum, so I have an idea what the percept would be like. Insects can see UV too, I think, so surely thats a colour for them?

    • Stephen says:

      I wonder how far into the UV they can see and how the molecular basis of that extended sensitivity is achieved…?

      • Steve J says:

        Well I know that they are sensitive to insect control devices fitted with an UV-A 315-400nm lamp. You could try an experiment with UV-B and UV-C lamps too, but I wouldn’t want to be too near them!

  7. biochembelle says:

    What a fantastic response (even with the semantics of how ‘color’, excuse me, ‘colour’ is defined). 🙂

    I am not a structural biologist, but I do have a keen interest in how molecular interactions manipulate biology. Recently I was putting together a journal club presentation on the train to work and had a structure up in a molecular graphics program. The woman sitting next to me asked what it was. I explained that they were proteins, sort of machines in cells. She commented on how beautiful it was, incredible that it can be done, and asked what I worked on. Sometimes all it takes is a visually stimulating image to capture the attention even of an adult.

    • Stephen says:

      Yes I have been surprised by the response.

      To think I nearly didn’t bother to post this… but ultimately my aim was to try to find a way into talking more about the molecular side of life. Though the discussion has been a bit hung up on my (admittedly loose) use of the word colour, I would like to explore ways of getting more publicity for my beloved protein structures. I think there is a fascination out there — as you found with your neighbour — but too few points of entry.

      • I’m glad you did post it! I love this kind of thing. Exactly what I was trying to do by explaining genome sequencing by comparing it with Shakespeare (that sounds really silly written down out of context, but you know what I mean).

  8. Heather says:

    Ditto on the kudos for your post. We have too few ways to convey the excitement and beauty of what we encounter each day. Any way is a good way in. If nothing else gets through in my talks, I regularly get a “you’re so *enthusiastic*” which usually makes my day (of course, a real live scientific discussion would make my day more, but those are less frequent).

    My favorite color by conventional definition is due right of the center and on the edge. Or at least, it always had been since I was little. Perhaps things are getting more nuanced with age. I have a friend whose favorite color is certainly radio, so I suppose that puts him quite a ways over the same edge.

    • Stephen says:

      Yep – I’m a fan of radio too but, in contrast, not so keen on UV. With my pale northern European complexion, it brings me out in a nasty shade of red.

  9. Steve Caplan says:

    Great post! I’m with you–the need is there to convince the public at large about the significance of basic science including structural biology. We need more advocates like you, who direct films on scientists and show the beauty of X-ray structures!


  10. Katie says:

    Brilliant post, whether or not x-ray is really a *colour* isn’t the point, this is an excellent way-in to discussing and explaining how we are able to see into the mechanisms of our molecular biology, and how completely incredible and exciting that is.

  11. Duncan Hull says:

    Hi Stephen, I liked this post but it messes with my head…

    Kestrels can see near-UV light but we don’t classify UV as visible light. Which highlights the fact that its a rather anthropocentric term and makes me wonder, what colour do kestrels perceive (if any) when the “see” near UV?

    It messes with my head to think that light is both an electric and a magnetic field. Can you explain that to kids too (without messing with their heads)?!

    • Stephen says:

      Ha – very good point about the kestrels. Now, all I need to do is find an animal that really sees X-rays… (or start applying some evolutionary pressure for one).

      The EM question is trickier if anything. Classically, the variation in the electric field is due to movements of a charge and charge movement necessarily also generates a magnetic field. To ‘see’ that, you probably need to have a decent understanding of Maxwell’s equations and to get on top of them is quite a journey.

  12. I’m also reminded that some photographers enjoy infrared photography, which is of course cheating. Either film that is sensitive to IR, or a digital camera with the filter in front of its sensor removed (that filter there specifically to block IR) is used to “see” IR – but of course what we see on the screen or printed page is a visible-colour interpretation of what the IR might sort-of look like.

    That said, black and white IR photography can be simply stunning. The Wikipedia article linked above doesn’t nearly do it justice, but there are lots of examples to be found with a bit of Googling.

  13. P.S. Nominated for OpenLab 2013. 🙂

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  15. MaryUYSEG says:

    What a great discussion from an interesting starting post.
    Thank you.

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