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.
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
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.
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.