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.

Optics and Life: Strange Sight - The world in Ultra Violet

We are all familiar with rainbows, showing us the full spectrum of colour that we can see- red, orange, yellow, green, blue, indogo and violet, but the electromagnetic spectrum continues beyond both sides of the rainbow. Red is the longer wavelength (around 600nm), and longer we have infra-red (which is pretty much responsible for the radiated heat you feel from a fire or the Sun), microwaves and the longest - radio waves. Beyond violet we have ultra violet (UV), x-rays and gamma rays.

As you can see, the actual bit of the electromagnetic spectrum that we can see is very narrow. What would it be like if we could see beyond our limited range?

Well as a matter of fact, many organisms can. Indeed it is often an essential part of their lives. Pollinating insects such as bees can see into the Ultra Violet, and it is for visibility to bees that flowers have co-evolved their colours (along with the insects ability to discern them). But you might ask - if bees can see into the UV, then what do flowers look like to them? Well often they are very different indeed, here are a few examples:

This is the common dandlion - on the left is the normal visible light image, and on the right is the UV image. This is colour shifted so we can see it, but nevertheless shows us that there is a strong two tone image, with the bright part in the middle of the flower, telling the bees where the nectar is.

This one is an evening primrose. Again yellow to us, but the insects can see lines, almost like landing strips on the runway, pointing to the pollen and nectar in the center.

UV photography does require special equipment. Firstly, you need to be able to cut out the visible light using filters, and then you need detectors that are capable of imaging the UV light, and you also need lenses that can focus the light. More information can be found here:

http://photographyoftheinvisibleworld.blogspot.com/

Other organisms can see into the Infra-red. This is particularly useful, because water does not absorb infra red light so easily as other wavelengths, and so the fish can see further.

Some can even see different polarizations of light - again many bees and insects. This is particularly useful, as it allows them to see what direction they are going in, and possibly even see predators underwater.