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:

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.