Quickfire Question: How do LEDs work?

LEDs, or Light Emitting Diodes are very common devices used in a wide variety of applications from some street signage, power indicators, transmitters in remote controls and even LED torches. They are very efficient devices, which much like sodium lamps, convert most of the current passing through them into light, with very little loss as heat, but how do they work?

All LEDs are made from semiconductor materials - materials which have conducting properties somewhere between insulators (like glass) and conductors (like metals). Semiconductors can be carefully constructed to perform a variety of applications, such as diodes, which only allow current to pass through in one direction, to transistors - which either allow current to pass, or stop it, depending on the voltage at a "gate", solar cells and much more elaborate structures ranging from logic circuits all the way up to computer chips.

Like the previously mentioned diodes, LEDs only allow current to pass through in one direction, and when the current passes through, light is emitted. There are a couple of ways that LEDs can be constructed, I will concentrate on the simplest.

Semiconductors can be "doped" with other materials, which can either donate electrons (n-type semiconductors), or can accept electrons (p-type semiconductors). The former have extra electrons which can flow through the semiconductor from the negative to the positive terminal, and the latter have "holes" which are like positively charged electrons, that flow from the positive to the negative terminal. When the two meet they can recombine, and release energy in the form of light.

By varying the dopants, we can manipulate the wavelength of light that is emitted. Other methods of varying the colour are more elaborate, and involve the use of quantum tunneling, different sorts of junctions, and even adding additional materials. As we can see, the spectrum of conventional LEDs tends to be very pure:

producing a very limited spread of colour in each LED, however the addition of other chemicals such as phosphors, can "down convert" high frequency light such as blue, and re-emit that light in a broader spread of wavelengths. This is a common method of producing white LEDs.

More recently another type of LED has been developed, known as an Organic LED, or OLED. In place of inorganic materials such as Indium and Gallium, OLEDs use carbon based chemicals (hence organic) that emit light. The semiconductor properties of these materials are similar, though the emission is somewhat different, having a much broader spectrum. Some of the details and issues surrounding OLEDs will be covered in a later post.

Light and the Age of the Universe - the Cosmic Microwave Background

Our main window to understanding the universe is light and the electromagnetic spectrum. Trapped here on earth, there is very little of the universe that we can actually touch and test with our own hands, but light provides an amazing tool. The Cosmic Microwave Background is perhaps on of the best methods we have of finding the age of the universe.

All objects that are in thermal equilibrium - that is, the matter and EM radiation in the objects are the same temperature - have what is known as a black body spectrum - EM radiation with properties that are a function of the temperature of that object only. That spectrum might be modified a little but atomic absorption and emission lines, but the fundamental black body spectrum will remain. The spectrum looks like the curves on this graph:

Each curve represents a black body emitter with a particular temperature, shown in Kelvin (roughly the temperature in degrees plus 273, where 0 is absolute zero). The Sun, indeed all stars have a black body spectrum. In the case of the sun, the surface, and black body temperature is about 6000K, so it looks not so dissimilar from the 5000K curve. You can see that the black body spectrum continues beyond the visible - indeed the IR part is what is responsible for heat from the sun. The earth has a black body spectrum of about 278K (5.5 Celsius), which peaks in the infra red.

So what does this have to do with the age of the universe? Well when the universe was a mere 400,000 years old, about 13.7 billion years ago, everything was very much closer together, though space was expanding rapidly, and so the universe was much hotter than it is now, so hot in fact that there were no atoms, there was just a sea or plasma of hydrogen and helium nuclei (and a bit of lithium) electrons, Electromagnetic radiation and other subatomic particles (earlier than this there weren't even nuclei, but that's earlier than we are interested in here). The universe was still too hot for the electrons to bind to the nuclei, and so photons were constantly being absorbed and re-emitted by the various charged particles that were around, and the universe was in a state of equilibrium between matter and radiation. This means there was a black body spectrum. Eventually, as the universe expanded electrons no longer had enough energy to constantly escape binding to the nuclei, and they finally bound, becoming hydrogen and helium atoms. There was still substantial interaction between matter and radiation, particularly in the form of scattering, such as Compton Scattering and Thompson Scattering. The universe continued to cool as it expanded further, and eventually cooled down to a temperature of about 4000K at which the scattering dropped off. The radiation at this point became decoupled from the matter in the universe, as the universe became transparent, though the shape of the spectrum remained imprinted on the light that passed on, and continued traveling through the universe.

In the intervening billions of years, space itself continued to stretch. Imagine drawing a wave on a balloon, and blowing up the balloon. You will see the wavelength becomes longer and longer. The same effect occurs to the radiation, but now the the very space of the universe is expanding, so photons that initially had a short wavelength, over time were stretched out so the wavelength was longer and longer, so long in fact, that the BB spectrum which peaked at 4000K now peaks at a temperature of just 2.725K - barely above absolute zero.

This temperature is the same in all directions, though there are tiny fluctuations, which resulted from small changes to the very uniform distribution of matter and energy in the early universe as we can see in the (Wilkinson Microwave Anisotropy Probe ) WMAP satellite image below. It is these tiny imperfections that seeded the collapse of the primordial matter into the stars and galaxies of the universe today.

By knowing the rate at which the universe is expanding, which we can measure from the red shift of other features of the universe such as distant stars, galaxies and quasars, and these initial temperatures (which we know from looking at hydrogen and helium in the lab) we can then deduce the age of the universe (in much the same way as we can determine how long a cup of water has been standing on a table for if we know it was boiling when it was put there)...

...13.7 billion years old.

Quickfire Question: How do incandescent (filament) bulbs work?

We are all familiar with incandescent bulbs, which have been until relatively recently been the most popular sort of bulb.

A voltage is placed across a metal filament held in an inert gas like argon, neon or nitrogen, in order to stop the gas from reacting with the filament and allowing the bulb to live longer. The filament has a high resistance to the current flowing through it, and this heats the filament, causing the atoms in the filament to vibrate. As the atoms vibrate, they then radiate energy in the form of light.

An important point that can be made here, is that all vibrating atoms and molecules will radiate somewhere on the electromagnetic spectrum. The hotter they are, the faster they vibrate, and thus the higher the energy (and frequency) the photons are that they emit.

A problem with this sort of lamp, is that they are very inefficient. Because the light is generated by heating, large amounts of energy is lost in unwanted heat. Also as the filament is heated, it slowly evaporates over time and eventually breaks, leading to a relatively short lifetime. It is mainly for these reasons that there is a move to using energy saving bulbs, which both have a longer life, and also produce the same amount of light for less energy input.

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.

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.

The Ancient History of Optics

People have puzzled and pondered over how we see for many thousands of years, though a more complete understanding of optics was not truly available to us until more recent centuries through the work of scientists like Newton, Young, Maxwell and Dirac. Some of the earliest known writings on optics date back to the Ancient Greeks. The majority of the earliest ideas on optics were largely speculation. In the fifth century BC for example, the greek philosopher Empedocles hypothesized that the eye contained fire, which shone out from the eye illuminating objects so that they could be seen. Of course this raises many questions, such as "why can't we see at night then" and "why don't objects appear incredibly bright when several people look at them" - for the former question Empedocles thought that there may be some interaction between the rays from the eyes, and rays from a source like the Sun or a candle. Unfortunately the concept of Occam's Razor (entia non sunt multiplicanda praeter necessitatem - or entities must not be multiplied beyond necessity) wasn't devised until the 14th century, or Empedocles' contemporaries may have suggested doing away with the eye-beams and just leaving the rays from bright objects like the Sun. Still, it was relatively early days, and there were many great thinkers to come, who would apply their minds to the problem of light and vision.

To backtrack a little here, while there was no theoretical understanding of light and optics, People had still made practical applications of optical components and light itself. One of the earliest known lenses for example was the Nimrud lens, shown here - a piece of shaped glass some 3000 years old in the remains of the ancient city of Nimrud, which lies within modern-day Iraq. This lens was discovered by Austen Henry Layard in the mid 19th century, and may have been used as a magnifying lens, either for looking at objects or starting fires. Many similar lenses exist through the ancient Greek, Roman, Babylonian and Egyptian cultures. These though tended to be carved from crystal or glass spheres filled with water.

Back on to the theory. A couple of hundred years after Empedocles speculated on how vision worked, Euclid wrote Optics, which was one of the first texts to study the geometrics of optical systems in more mathematical detail. Ptolemy, this time a Roman Citizen (though possibly of greek ancestry) who lived between the first and second centuries BC, extended this work futher. His writings are considered some of the most important writings on optics before Newton, although they survive only as translations into Arabic - the originals having been lost. In this, he introduces many of the important properties of optical systems, talking about light, refraction, reflection and colour (all things I will get on to later)

Not all of these early studies were limited to the Greek and Roman empires though, much important early work was also carried out by Arabic scholars such as Ibn Sahl (10th century BC) who discovered the law of refraction (now known as Snell's law) and Ibn Al-Haytham (10th-11th century BC) who did away with Empedocles' rays from the eyes and more carefully defined what the rays were.

Things lay relatively quiet on the optics front then, until the Rennaisance when scientists such as Johannes Kepler and Willebrord Snellius began investigating the mathematical and physical behaviour of light. That though, will have to wait until later.