Sunday, February 28, 2010

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.

Saturday, February 27, 2010

The Science of Optics: Polarization of light, Water, Insects and 3D Cinema.

As mentioned earlier, light is a transverse wave, but as it is in 3D space (rather than on a surface like water) it can oscillate in any direction perpendicular to the direction of motion. The particular polarization depends on the relationship between the electric and magnetic field, but the simplest polarization is linear polarization - the light oscillates in a flat plane. When interacting with materials, it is often useful to be able to determine the relationship of the polarization to the material or angle of reflection. Here we will consider reflection from a mirror.

The light travels in the direction of the black line, and bounces off the mirror (the grey shape) - so the light takes a path that stays in the plane of the blue shape. The blue wave oscillates perpendicularly to the blue shape and is known as perpendicular or s-polarization. The red wave oscillates in the plane of the blue shape and is known as parallel or p-polarization. The light can also oscillate in any direction perpendicular to the direction of travel, and so some component of the light can be perpendicular, and some can be parallel.

This is very important in reflections, because different amounts of the perpendicular and parallel components can be reflected from a surface. The light that reflects from water at a shallow angle for example is almost all p-polarized light. That means if we take a polarizing sheet or polarized glasses (remember it has to be linear polarization - 3D glasses from cinemas are usually circularly polarized, so this won't work) and hold them in front of the reflection, we can cut out almost all of the reflected light and see into the water.

This image shows two photos of a puddle - one without a polarizer and one with a polarizer. The polarizer removes all of the light reflected from the surface, and so the reflection of the building disappears.

Interestingly, when locusts are swarming, they avoid areas of ground where there are large amounts of horizontally polarized light, because that means the light is reflected from water, meaning they avoid lakes and only land where there is food. You can read more about that here.

Some scattered light is also polarized, particularly light that is Rayleigh scattered. Rayleigh scattering occurs when the object that the light scatters from is very much smaller than the wavelength of light. Rayleigh scattering is stronger for shorter (bluer) wavelengths of light than for longer (redder) wavelengths. A good example of this is the scattering of sunlight that makes the sky blue.

As sunlight passes through the atmosphere, more of the blue light is scattered than the longer wavelengths, and so the sky appears blue. Just like the reflection from the water, this light is also partially polarized (though not totally, because of multiple reflections that can mess the polarizations up a bit). The polarization of the sky is in a direction that is tangental to a circle drawn around the sun.

As a result of this, insects which can detect the polarization of the light can tell where the sun is in the sky, even on cloudy days, and without being able to see the sun or shadows. Since this polarization follows the sun as it moves through the sky, this allows insects like bees to find the same patch of flowers even as the day goes on.

Circular Polarization and 3D Cinemas

So far I have described linear polarization, but light can also be circularly polarized. If we imagine some light traveling in the x direction, oscillating at an angle between the y and z direction, we can project its components in the y and z direction like this:

As we can see, they are in phase. This means they are doing the same thing i.e. they are both maximum at the same time, zero at the same time and minimum at the same time. But what happens if they are out of phase?

When we add them together, we can see that the electric field now rotates around the x-direction. This is known as circularly polarized light. The light can either spin clockwise as it moves, or counter-clockwise. Just like with the linear polarizer, we can have polarizers that let through only one circular polarization of light and block the other, and this is the technique that some 3D cinemas use - One lens blocks light that is clockwise polarized, and one lens blocks light that is counterclockwise polarized. This means that different images can be sent to each eye, and then your brain can make a 3D image from these.

Circular polarization is used rather than linear polarization, because if one image was projected using horizontally polarized light, and the other using vertically polarized light, the glasses would have to be perfectly oriented all the time, or you could keep picking up a bit of the wrong image in your eyes, making you see a double image (like you see if you take the glasses off). Circularly polarized light is not affected in this way. Note that only some 3D cinemas use this technique - others have switching glasses, that very rapidly block and unblock the eye, allowing your eye to see alternate images.

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:

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.

Wednesday, February 10, 2010


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.

Tuesday, February 9, 2010

The Science of Optics: What is a Photon?

Once scientists started developing a better understanding of how light works - how it propagates and its effects. One of the biggest mysteries has been - what is it exactly? Is it a wave? Is it a particle? It turns out that it is in some ways neither, and in some ways both, and now I will explain why we know this.

The Wavelike properties of light

The wavelike properties of light are just like water waves - although they are not attached to a surface like water. They are transverse waves, which means the oscillation occurs at right angles to the direction the wave travels in, like in water, where the wave oscillates up and down, but the wave appears to travel along the surface. This is unlike sound waves which are longitudinal waves. In these sorts of waves, the oscillation occurs in the direction of the wave movement, like along a slinky spring.

How do we know that light is a wave? Well to test this, we can use a property known as interference. If we have two sources of waves that are correlated with one another - that is they are both of the same frequency and start off in phase, then there are a number of experiments that we can do. The simplest of these experiments is the Young's Double Slit experiment.

In this we have two slits that act as the sources and we shine light through them. The light is then projected from there on to a screen and we observe the pattern on the screen. When the path length is the same, the light arrives in phase and we get constructive interference - the two waves add up and we get a bright line. Where the path lengths are half a wavelength out of phase (in the water, this would be where a wave peak meets a wave trough) we get destructive interference and we get a dark line. If we know a few facts like the distance from the screen to the slits, the distances between the slits and the distance between the bright fringes, then we can do a bit of trigonometry and work out the wavelength. A more detailed explanation can be found on wikipedia's page on the Double-Slit experiment.

Interference can only occur where we have waves, so we know that light is a wave.

The Particulate nature of light

If light is a wave, then as the wave expands out into space, the energy will become ever thinner along with it, because energy is conserved. The wavelength however will change. For a water wave, the energy of the wave is related to its amplitude - the bigger the amplitude, the bigger the wave and the more energy. Just look out at sea - you need a pretty big wave with a lot of energy to throw an oil tanker around, but yachts can be thrown around by much smaller waves.

If we shine light onto a metal, then under certain conditions, electrons are thrown off the metal. This is because the electrons absorb the light, and if they have enough energy to escape the surface, they can jump off, in a sense like boiling a kettle where if the water has enough energy, it can leave the liquid and become a gas. This is called the photoelectric effect. Investigations into the conditions of the photoelectric effect threw up a rather strange phenomenon, and eventually it was Einstein who explained it.

It was noticed that if we shine really intense light on to a metal, and then measure the energy of the electrons coming off it, the energy of these electrons was the same as if we shone much dimmer light of the same frequency. The intensity is like the amplitude I mentioned earlier, so obviously this is pretty weird. Imagine our yacht going over a wave, for a really big wave, our unfortunate captain might be flung overboard, but for a little wave he can stay on the deck sipping his tea and enjoying the cruise, but for electrons, this isn't what happened at all. Regardless of the amplitude, the electrons were all flung from the surface with the same energy. What did change though, was the number of electrons that were thrown from the surface.
When the frequency of the light was changed, it was found that the higher the frequency, the more energy the electrons had when they were flung off. If the frequency was below a certain value, then no electrons would be thrown off at all, regardless of how bright the light was.

Einstein was the first to explain this in 1905, in his Nobel Prize winning paper "On a Heuristic Viewpoint Concerning the Production and Transformation of Light". The idea he presented, was that light came in discrete packets that he called "light quanta". The amplitude of light related to the number of these quanta, but the energy of each quanta was related to its frequency. Electrons could absorb one of these quanta and if there was enough energy to escape the attraction of the surface then they could, with the remaining energy going into the kinetic energy of the electron.

This explains the results perfectly - increased amplitude, or numbers of these quanta results in more electrons of the same energy being thrown off the surface, and increased frequency results in the electrons having more energy.

So we now know that light consists of quanta, or particles.

But hold on, we already said that light was a wave, and we demonstrated it with experiments. Now we are saying that light is a particle, and also we demonstrated it with experiments. Scientifically we can alter the theory, but we can't just toss out facts we don't like, so what is going on here? Enter Quantum Mechanics.

In Quantum mechanics, things may have both wavelike and particulate properties. It doesn't really make sense to ask whether something is a wave or a particle. Those after all are just descriptions of things that we are familiar with, that have certain sets of properties. Quantum Mechanics shows us that all things have properties that are both wavelike and particle like. I'll stick with photons here, since that's what I'm talking about, and it's what this blog is all about, but the best way to describe this, is that photons propagate in a manner that is wave like (they spread out over space), but interact in a way that is particle like (they interact discretely). This of course raises loads more interesting questions, like "well where is the photon if it is spread out all over the place but only interacts once, and in one place" but for now, this will have to do. If you do want to confuse people though, if someone happens to ask "what is a photon" a good concise description of it would be "a quantized excitation of the electromagnetic field" - I will get on to the electromagnetic field another time.

A much more in depth explanation of the photon and the history of the photon can be found in W.E Lamb's paper "Anti Photon, Applied Physics B, vol 60 p77-84 (1995).