Skip to main content

What colour is an electron?

What colour is a beam of blue light?
Not long ago I facetiously commented on Facebook that electrons were pink. The next day, an X-ray crystallographer asked me 'As someone with a physics background, what colour would you say an electron is?' I almost fell off my chair. But once I started to think about it, it's a really interesting question - and one that might be worth first approaching by asking another apparently silly question.

What colour is a beam of blue light? The answer certainly doesn't have to be blue. Before I explain why, let's put relativity out of bounds. Once you start moving, colours are moveable feasts - think blue/red shift. But I'm envisaging a much simpler situation. I show you a beam of blue light and ask you what colour it is. I can guarantee you would not answer 'blue'.

To avoid distraction, what I will do is shine the blue light down a cylinder with a black interior, turn the lights off in the room and open a door on the side of the cylinder so you can see the light passing through. What would you see? Nothing. Because in one sense you can't see light. Obviously this sounds bonkers. Light is all we do see. But the point is that when we see that we see light we mean something totally different to seeing, say, a postbox. When I say I see a postbox, what I mean is that the light from, say, the sun, hits the postbox, is re-emitted by the box towards my eyes, and I see the box. So 'seeing' usually means detecting the light that hits an object and comes back towards us. This just doesn't happen with light. Light passes right through a beam of light - it doesn't reflect off it. So we don't see a light beam sideways on.

You may at this point be thinking, 'but what about laser beams and spotlight beams and such? You see those sideways on.' And as the picture above demonstrates, you do. But only because the beam is hitting something in its path - dust, water vapour or smoke, for instance - and some of the photons are being scattered off their path towards your eyes. Otherwise there would be nothing to see.

So bearing this in mind, let's go back to 'What colour is an electron?' My initial thought in response to the crystallographer was that it doesn't have a colour as you could either consider it a dimensionless point or a spread out, fuzzy quantum collection of probabilities, and in both cases the concept of colour is meaningless. But he had something different in mind.

He said that physicists usually think of electrons as blue (perhaps as a result of the Čerenkov radiation given off by electrons in nuclear power plants), and tend to think of positively charged things as red, which is the opposite of the convention he used. He was just talking about arbitrary conventions. But now that I come back to the concept given the insight from 'What colour is a beam of blue light?' I think that maybe there is a real answer.

When we see a postbox as red, what is happening is that the box is absorbing photons from the sun with a range of energies corresponding to the whole visual spectrum (and beyond). Much of the energy from the photons simply goes into increasing the energy of the atoms in the box (effectively warming it up a little), but some of the energy is re-emitted as photons, preferentially in the red range, so we see the box as red. 'What colour is a postbox?' really means 'What energy range of photons are re-emitted by the box?'

Specifically, the particles responsible for that re-emitting are electrons. When light hits an object, the electrons around the atoms in the object absorb the light energy, jumping up one or more levels. The light that comes back off the object, enabling us to see it, is the result of those electrons dropping back down in energy, releasing a new photon or photons. So arguably the colour of the electron is the colour of the light it re-emits. This varies depending on the electron's state - so  you could argue that the real answer to 'What colour is an electron?' is not 'It doesn't have a colour,' but rather 'An electron is a bit like a chameleon. It has different colours depending on the state and situation it is in.'

There's nothing like a silly question to get the brain in action.

Image from Wikipedia


Popular posts from this blog

Is 5x3 the same as 3x5?

The Internet has gone mildly bonkers over a child in America who was marked down in a test because when asked to work out 5x3 by repeated addition he/she used 5+5+5 instead of 3+3+3+3+3. Those who support the teacher say that 5x3 means 'five lots of 3' where the complainants say that 'times' is commutative (reversible) so the distinction is meaningless as 5x3 and 3x5 are indistinguishable. It's certainly true that not all mathematical operations are commutative. I think we are all comfortable that 5-3 is not the same as 3-5.  However. This not true of multiplication (of numbers). And so if there is to be any distinction, it has to be in the use of English to interpret the 'x' sign. Unfortunately, even here there is no logical way of coming up with a definitive answer. I suspect most primary school teachers would expands 'times' as 'lots of' as mentioned above. So we get 5 x 3 as '5 lots of 3'. Unfortunately that only wor

Why I hate opera

If I'm honest, the title of this post is an exaggeration to make a point. I don't really hate opera. There are a couple of operas - notably Monteverdi's Incoranazione di Poppea and Purcell's Dido & Aeneas - that I quite like. But what I do find truly sickening is the reverence with which opera is treated, as if it were some particularly great art form. Nowhere was this more obvious than in ITV's recent gut-wrenchingly awful series Pop Star to Opera Star , where the likes of Alan Tichmarsh treated the real opera singers as if they were fragile pieces on Antiques Roadshow, and the music as if it were a gift of the gods. In my opinion - and I know not everyone agrees - opera is: Mediocre music Melodramatic plots Amateurishly hammy acting A forced and unpleasant singing style Ridiculously over-supported by public funds I won't even bother to go into any detail on the plots and the acting - this is just self-evident. But the other aspects need some ex

Mirror, mirror

A little while ago I had the pleasure of giving a talk at the Royal Institution in London - arguably the greatest location for science communication in the UK. At one point in the talk, I put this photograph on the screen, which for some reason caused some amusement in the audience. But the photo was illustrating a serious point: the odd nature of mirror reflections. I remember back at school being puzzled by a challenge from one of our teachers - why does a mirror swap left and right, but not top and bottom? Clearly there's nothing special about the mirror itself in that direction - if there were, rotating the mirror would change the image. The most immediately obvious 'special' thing about the horizontal direction is that the observer has two eyes oriented in that direction - but it's not as if things change if you close one eye. In reality, the distinction is much more interesting - we fool ourselves into thinking that the image behind the mirror is what's on ou