Quickfire Question: Why are street lamps amber?

We are familiar with the colour of many street lamps, the amber glow of the sodium vapour:

But how do these lamps work, why are they the colour that they are, and why do we use them?

(1) How do Sodium Lamps Work

Inside the tube, there is a small amount of sodium metal, with a little neon and argon gas in there. A voltage is applied across the tube, and this excites the outer electrons in the neon and argon gas warming the sodium and also being responsible for the faint red glow that you can see before the light turns on fully. Eventually the sodium vaporizes and due to the voltage, its outer electrons too are excited. When those electrons spontaneously drop back into their ground state, they emit light at a very particular wavelength - the amber light that we see.

(2) Why are Sodium Lamps that colour?

Only one electron is excited by the electrical discharge, and only with enough energy to jump up one energy level, so when they drop back to the ground state they can only emit a very pure colour of light. The wavelength of sodium light is 589.3nm. (actually there is a little more going on, meaning there are two lines very close to one another at 589.0 and 589.6nm). This is known as monochromatic light, and it is also the reason it is hard to pick out any colour of objects lit by a sodium lamp alone.

(3) Why do we use sodium lamps?

There are a number of reasons, and I will outline a few: Sodium lamps are very efficient, because most of the energy is turned straight into light, unlike incandescent filament lamps, which turn a lot of the energy into heat. That wavelength is also pretty close to the optimal response of the human eye:

This graph (called the scotopic response curve) shows how the eye responds (side axis) to different wavelengths of light (on the bottom axis). We can see the sodium lamp is pretty close to the middle. That means that a lower level of light is needed to see things clearly.

They also use no mercury or dangerous metals, and so are easily disposed of, keeping the cost down even further.

Lasers

We are all pretty familiar with lasers these days, from laser pens to the sorts of lasers that evil masterminds use to cut British Secret agents in half (an example here being James Bond and Goldfinger), but what are they exactly?

Laser is actually an acronym, and it stands for Light Amplification by Stimulated Emission of Radiation. let's go through the terms. Light... well we know about that. Amplification - makes things brighter - easy so far. Radiation - another word for the electromagnetic spectrum, which includes light, so again, no problem. But what about Stimulated Emission? what does that mean? To explain this, I will start off with spontaneous emission in atoms. This also works with molecules as well, but atoms are a little easier to explain.

Normally atoms rest in their ground state, all the electrons in the atom are as low as they can be. The energy levels in atoms are limited, so if we imagine them as shelves, only two electrons can go on the bottom shelf, eight on the next, eighteen on the next and so on, with as many electrons in the atom as there are protons. Now when an atom becomes excited either by being heated or when it absorbs a photon, one or more of these electrons can jump into a higher shell (I will refer the difference in energy between the lower and upper level as the energy gap) . Left on its own, the atom may only stay in this state for a limited length of time, after which the electron will then drop down again to the ground state, emitting a photon along the way. The energy of the photon is the same as the energy gap. The important thing here though, is that length of time - it is only an average and is quite random. The electron spontaneously decays to the ground state, without any influence, and the emission of the photon in this case is called spontaneous emission.

Now imagine that we have our atom in its excited state, just like before, only this time, the atom is hit by some electromagnetic radiation of the same energy as this energy gap. This can jiggle the atom and force the electron to decay back to its ground state, so now we have two photons which can then go on to hit a couple more excited atoms, and we have four photons and so on. This kind of forced decay of electrons into their ground state is called stimulated emission. The stimulated emission is usually seeded by some spontaneous emission occurring in the laser.

There are limits to this of course; for one we need to have lots of excited atoms, and once we run out of excited atoms then we can't have any more stimulated emission. To get the excited atoms in the first place we have to dump lots of energy into the lasing material, and we do this in a technique known as pumping. There are lots of ways to pump a laser, but they all amount to pretty much the same thing - dumping lots of energy into the laser material, so that when photons pass through the cavity, they can stimulate the emission of more photons. The lasing medium itself has to be something that we can excite relatively easily, and there are lots of different materials such as ruby, special glasses (like Yttrium Aluminium Garnet), dye lasers and even gas lasers like Helium Neon (HeNe) lasers and Argon Ion lasers. Probably the most familiar sorts of lasers to us now are semiconductor lasers, used everywhere from DVD players to laser pointers.

A special feature of this light is that it is coherent. If we remember the previous post about the wave properties of light, coherent means that all the photons of one frequency are oscillating in step with one another. That means they always constructively interfere. This is unlike light from a normal fluorescent or incandescent bulb, which emits incoherent light (by spontaneous emission). It is this coherence that makes lasers so powerful, and why you have to be extremely careful when using a 4W laser like an Argon Ion laser (you have to wear special glasses and even stray reflections can burn your skin), even though the actual energy it emits is far less than a 60W bulb..

Laser Cavities

To make the laser more useful and to form a stronger pulse or beam, we can take our laser pump material and stick mirrors on the ends. One of the mirrors reflects all the light, and one of them reflects almost all of the light - usually something like 99%. Now the light can bounce backwards and forwards in the cavity. These mirrors are often specially shaped to make the beam as stable as possible. You do have to be careful here though, as if the amount of energy in the cavity gets too high - the light gets too bright - then you can start to get odd effects happening in the cavity such as self focussing, which can easily blow a hole in the lasing material. For glasses of course this will break the laser and the pump material will need replacing, and this presents a problem for high powered lasers.

Hopefully this has given you an idea about the basics of lasers. I haven't delved into any of the mathematics here, but you may be unsuprised to know that Einstein was a pivotal figure here once again, as he determined that stimulated emission had to occur.