Question: Why are streetlights orange?

Nathan Langford answered on 27 Jun 2012:

Hi Sizzle1999,

Great question! The reason why a lot of streetlights are orange has a lot to do with fluorescence, which is the process that creates all light in the universe. A lot of streetlights, particularly the older ones, contain sodium gas… sodium is just the stuff that’s in table salt (sodium chloride). Whenever you make light with something containing sodium, it will give off a yellow colour – I’ll tell you a bit more about why below. You’ll probably do some similar experiments in your science classes at school. Try putting a piece of copper wire in a flame, say a bunsen burner, and see what colour it makes. It turns out that different elements will give off different colours and this is in fact one of the ways scientists first used to try and identify what different substances were made out of – it was often called a flame test.

It turns out that the reason why different elements give off different colours is all because of quantum physics. If you’d like to know a bit more detail about that, I’ll explain it a bit more below.

Cheers,
Nathan.

ALL ABOUT FLUORESCENCE, ORANGE LIGHTS AND THE MAGIC OF QUANTUM PHYSICS

One of the most exciting things about quantum physics when it was first discovered was that the energy of small particles only comes in certain little amounts. This is quite a strange idea… If I throw a tennis ball to you, then the energy in the tennis ball is just how hard I throw it. I can throw it at any speed, from very slowly all the way up to very fast and everything in between.

But it turns out that individual atoms don’t behave like this – they’re more like a bookshelf. When you try and put a book on a book shelf, you can only put it on one of the shelves, say shelf number 2 or shelf 3. You can’t put it half way between shelf 2 and shelf 3 and expect it to stay there.

Now I’m going to tell you one of the most important laws of physics in the universe: “Everything wants to be lazy!” This is called the Second Law of Thermodynamics. When I was a kid and Mum asked me to do the mowing, sometimes I used to try and use the 2nd Law as an excuse to get out of it, but for some reason it never worked!

Anyway, while this law doesn’t necessarily help you get out of doing the household chores, it does work for atoms. And what that means is that, if you think about the bookshelf idea, atoms want to be on the lowest possible bookshelf where there is space – this is called the atom’s “ground state”. But what happens if you come along and give an atom a kick of energy? Well, if the atom absorbs that energy, then it jumps up from the bottom shelf to one of the higher shelves. This is called an “excited state”.

So the atom is now jiggling around in this excited state with all this extra energy, but then we come back to the 2nd Law of Thermodynamics, which says that “everything wants to be – lazy!” And after a while, the atom’s going to say, “all this jiggling around is ridiculous – I’ve had enough” and it’s going to drop back down to a lower level. But all that energy has to go somewhere – it can’t just disappear – so what does it do? Well, it often comes out as a little packet of light, called a “photon”.

This process – giving a kick of energy to an atom and waiting for it to drop back down to the ground state and give off (“emit”) a photon – is called “fluorescence”… It’s exactly the same process that happens in a fluorescent light bulb. There are different types of fluorescence, related to how you put the energy in in the first place. For example, you can use “electrical discharges” or “sparks” (which are fast moving electrons), like in lightning strikes; you can use heat, like from a flame or in a normal light bulb; or you can even use more light, like when you see people’s clothes glow under UV.

But why do different substances give off different colour fluorescence? Well, the first thing to know is that the energy of a photon is related to its colour – that is, when we see photons with different energies, we “see” that as being light with different colours.

The second thing comes back to quantum physics again. Remember how I said that atoms are like bookshelves? Well, what quantum physics tells us is that *different* types of atoms are like *different* bookshelves where the shelves have different spacings between them. Some of these spacings will give off visible light and some of these spacings will give off light in the UV or in the infrared (both invisible to us). So all copper “bookshelves” (atoms) have shelves with one pattern of spaces and all sodium bookshelves have shelves with another pattern of spaces. And with sodium, it turns out that the main “shelf spacing” (which we call an “atomic line”), is one with a wavelength of around 588 nanometres, which is a yellowy-orange colour.

So we use sodium in streetlights partly because it’s a really common element (e.g., you can get it from normal salt) and partly because our eyes are quite sensitive to yellow light, so it looks really bright.