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.

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.

Let there be Light

Hi and thanks for taking a look at my blog. I'm an optical physicist, having studied optics for several years from a lowly undergraduate and Masters student at Imperial College London, before completing a PhD in Organic LED technology, and now working in Biomedical Sensors. The aim of this blog is going to be a combination of updates of the latest news in optics covering a wide variety of areas (just whatever catches my eye at the time) and explanations of the various principles of optics. It's intended for a lay audience, but I may get a bit technical at times - this is a good opportunity for you to learn!

I hope after reading some of these articles, you can learn something new, and understand a bit more about the Light side of Science.