Why are ultra-violet lamps used at parties and discos? Because they create an appealing effect that is different from illumination by normal white light. It’s not just that purple is a nice colour. You could use a purple light for that. It’s that white clothes look
(a) white, when they should look purple. I’ve written about colour before, here, and that post will help if you aren’t convinced the clothes ‘should’ look purple.
(b) implausibly bright according to our everyday sensory experience. They seem to be ‘glowing’.
So what’s going on?
First, let’s get a misconception out of the way. See how the ceiling looks purple (or violet)? That’s NOT ultra-violet. It’s violet. You can’t see ultra-violet – it’s invisible. The purple light you can see is just that – visible, purple light.
OK, so what’s a u-v lamp then?
Visible light
Light exists at different wavelengths (and different frequencies – higher frequencies correspond to shorter wavelengths). Different wavelengths of light correspond to different colours. Red light has a wavelength of approximately 700 nanometres (nm) – just under a thousandth of a millimetre. Violet light has a shorter wavelength – approximately 400 nm. The colours of the spectrum are in between (so when as a child you learned “Richard of York gave battle in vain,” as a mnemonic for the colours of the rainbow, the “Richard” and “Vain” are at the two ends of the sentence).
The electromagnetic spectrum
Those visible colours are not the whole story. We might think of ‘black’ as ‘no light’, but it would be better to think of it as ‘no visible light’. If you sit in a darkened room, you will not emit visible light yourself; you are not luminous. But you will emit infra-red radiation (i-r) – your ‘body heat’. That is exactly the same stuff as light, but with a wavelength that you can’t see. It has a wavelength greater than 700 nm. It exists in the left-hand ‘black’ region of the diagram above. Someone in the same room as you with a thermal imager would be able to see you. (In fact they still wouldn’t be able to see the infra-red you emit – it’s invisible! But the thermal imager will convert that infra-red into visible light they can see.)
The fact you can’t see infra-red is more a question of biology than of physics – your body is not adapted to sense it, but is adapted to sense the wavelengths that we call visible light. The wavelengths of visible light are just a small ‘window’ that we are sensitive to among a whole range of other wavelengths that we do not experience.
When it became clear that visible light was only part of a broader class of stuff, that broader class needed a name of its own. It became called the electromagnetic spectrum; i-r, visible light and u-v are all examples of electromagnetic waves. The infra-red region of that spectrum was the first to be discovered (other than the visible bit, which didn’t need discovering) in about 1800. Obviously, people knew about radiant heat before 1800, but they didn’t realise it is just light with non-visible wavelengths.
Ultra-violet
We have dealt with the black part of the diagram to the left. What about the black part to the right, beyond the violet? Well, yes – that’s the ultra-violet. You are not emitting ultra-violet, whether you are sitting in a darkened room or not. That requires more energy than your body possesses (short wavelength = high frequency = lots of energy needed to create it). But some things do create u-v. The Sun, for example. And u-v lamps.
But that ceiling in the picture is still reflecting visible violet. So what’s going on with the u-v lamps?
The first thing to say is that the purple light does come from the u-v lamp. The u-v lamp is designed to emit u-v (obviously), but like many lamps, it emits across a range of wavelengths. Some of those are in the u-v, but there is also some emission in the visible as purple light. So you can’t see most of the light coming from a u-v lamp, just the accidental bit of purple. That’s shown schematically in the diagram below…
Spectra
At first sight, the diagram above seems to show that we can see about one third of the light from the u-v lamp. But that’s due to an oversimplification in the diagram. The lamp will be emitting different quantities of energy at different wavelengths. We can transform the picture above easily into a graph to take that into account. It’s already a one-dimensional graph – from right to left is effectively a wavelength axis. If we make a vertical axis to show an amount of energy per unit wavelength per second (called the power spectral density), we might get something like this…
You can see that much less than one-third of the energy is radiated in the visible. That is because the wavelengths in the u-v are where the peak power is. The area under the curve tells gives you information about the power within wavelength ranges, and the blue/purple section is much less than a third of the total area under the spectrum.
And if that makes sense to you, you have just understood the scientific idea of a spectrum. When scientists describe a spectrum they often don’t just mean ‘the range of wavelengths present’. Instead they often mean ‘the energy distribution across different wavelengths’. Spectra crop up all over science, showing the wavelength-dependence of all sorts of things. If you ever see the word ‘spectroscopy’ and a picture with spikes and curves like we showed for the u-v lamp, then this may well be what is going on…
So back to the u-v disco. Which the cool kids are referring to as a glow party nowadays, apparently…
What’s with the white clothes?
First let’s think about what happens when we look at white clothes in daylight. You may have been told that white light is a mixture of all the colours of the spectrum. You are now capable of understanding this more precisely. Here is an approximate spectrum of (white) daylight – the light hitting the white clothes, together with the spectrum of the light reflected from the white clothes (this in particular, won’t be an accurate spectrum, but it serves the purpose of our explanation).
There is indeed energy emitted all across the band of wavelengths from red to violet (and outside that band too). But not equally. It peaks in the green/blue region, and tails off at either side. Anyway, when our eyes see light conforming to that spectrum, we see white…
The spectrum of reflected light has a total power (area under the curve) less than that of the light hitting the clothes. But that makes sense – the clothes can’t reflect more light than hits them. Looking at the areas under the two curves, I seem to have drawn this such that about three quarters of the light hitting the white clothes is reflected.
Now, what happens when we illuminate white clothes with u-v lamp? The clothes absorb the u-v, and re-radiate it in the visible. This process is called fluorescence, and is often caused by ‘optical brighteners’ that are deliberately put into clothes detergents for just this purpose. (Quinine makes a gin and tonic fluoresce under u-v, too!) As the molecules of these compounds absorb u-v photons, the photon energy goes into a mixture of:
- causing the molecule to vibrate
- causing the molecule to rotate
- exciting electrons from a lower to a higher energy state
The electrons mentioned in the third bullet will relax into the ground state and emit a photon, but the associated energy is less than was contained in the incident u-v photon, because some has been used up in the first two bullets. The upshot is that an inbound u-v (higher energy) photon has turned into an outbound visible (lower energy) photon. And that’s fluorescence (roughly).
So when we say that the clothes ‘should’ look purple, we are imagining a mechanism of reflection that leaves the wavelength of light unchanged (normal reflection). Fluorescence, though, is a different mechanism – the absorption of light at one wavelength (maybe in the u-v) and re-radiation at a longer wavelength. And if those re-radiated wavelengths cover a wide range of the visible spectrum, the appearance will be white. But what about the second weird thing about the clothes’ appearance – why so bright?
We can think about that in terms of the emitted and reflected spectra – they might look a bit like this.
Remember, your eyes can’t sense most of the u-v lamp emissions (the black part of the spectrum). So it appears as though the grey spectrum in the diagram has been created from just the blue/purple part of the u-v lamp spectrum. It appears that there is much more white light than should be available from the purple light creating it. These spectra show that that is not the case, that the black plus purple spectrum provides the energy for the grey spectrum, so that nothing is amiss. But your eyes/brain don’t see these spectra! They just see a very dim purple light seemingly producing a bright, shining white from the clothes.
So next time you find yourself in a club struggling to make polite conversation, you can launch into an explanation of the appearance of your surroundings!