Ch. 16 
Before going too much
further with this subject, it would be a good idea to quickly skim over the
introduction to this subject in Ch. 2 Waves. If you really like this sort of
thing, I highly recommend you get yourself a copy of the book Polarized
Light in Nature by G.P. Können. It
is a truly wonderful book, and the source of many of the figures and pictures
in this lecture. (I have my own personal copy).
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First, if the light waves have their electric fields all oscillating in the same direction, the light is said to be 100% plane polarized, meaning that all the oscillations are in one and only one plane. Light that has equal amounts oscillating in all possible planes is unpolarized. |
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If the light is not at one of these extreme cases, that is, there is some preferential direction that the electric fields are oscillating it, it is partly polarized.
Now, any such beam can be decomposed into a net oscillation in the x and y directions.
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This in turn can be thought of as the sum of a completely (100%) polarized beam and an unpolarized beam! Mathematically, they are equivalent. |
Now, for some more “fun”, it should be stated that this sort of decomposition into two orthogonal (perpendicular) waves works if the waves have the same wavelength and phase. That is, the crests are lined up so that they hit a target together. It is also possible to have light where the two components are 1/4 wave out of phase, so the electric field traces out a helix in space!
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Such a beam, shown here, would not be plane polarized, but circularly polarized. An observer would see the tip of the net electric vector trace out a circle instead of a line as it arrived. |
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And of course, light can
even be a mixture of circular and plane polarized elliptically polarized (totally or partially).
The way we study polarized light is to pass the light through some sort of optics whose properties are sensitive to the polarization state of the light. Polaroid filters are the most familiar tool to most people, and they have been incorporated into sunglasses.
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Because (as we will see), the light reflecting off most surfaces will tend to be partly polarized with a predominance of the electric fields oscillating horizontally, that “glare” can be diminished by having the transmission axis of the Polaroid filters aligned vertically. |
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However, we shall often be dealing with a variety of different orientations. |
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With polarized light, all we have to do is rotate the Polaroid filter to see of the brightness changes. If it does, the light is plane polarized.
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Remember this example from Chapter 2? This is a clear demonstration of the fact that in this direction of the sky, the skylight is polarized.
But the human eye is better at doing “comparative” measurements than “absolute” ones separated by time. One clever way around this is the use of a simple polariscope (usually called a Minnaert’s polariscope (M. Minnaert wrote the first book on the subject of light and color in the air over half a century ago, appropriately titled The Nature of Light and Color in the Open Air).
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It is made by taping a piece of cellophane onto a polaroid filter. The cellophane is an optically-active material that rotates the plane of polarization of the light that strikes it. If oriented correctly, it will rotate the light to its plane of polarization is orthogonal to the transmission axis of the filter. Light that is polarized in the direction of the transmission axis goes through the filter all by itself, but is blocked if it goes through the cellophane first.
The amount of rotation will actually depend on the wavelength of the light, and placing cellophane folded over between two Polaroid filters can give some interesting results!
The Sky
Sunlight scattering off of density fluctuation of air molecules (or off of dust) will be partly polarized. The polarization is strongest if the direction of the scattering is 90° from its original direction.
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At sunset or sunrise, the light is essentially tangentially polarized, but at a large angular distance it switches to radial. |
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In this “fisheye” (i.e. wide-angle) view of the sky we are looking through a Polaroid filter oriented with its transmission axis as shown by the arrow. |
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Light reflected off of and transmitted into a body of water can polarize light as well. And when we see a body of water in the daytime, the light that is striking it is already partly polarized. If the light hitting the water from a certain direction is vertically polarized, and the water most easily reflects any horizontal component, not much light will be reflected! |
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In those directions, the reflected sky will be darker than parts of the sky in other directions. |
One can preferentially darken certain parts of a sky, and even change the colors somewhat, when taking a photograph.
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When taking the picture below with the 35-mm SLR camera that I showed you in class, I inserted a properly-oriented Polaroid filter for the shot on the right.
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I always carried one of these in my camera bag!
The Ground
The ground reflects light too, of course. And in many cases, the light will be polarized. We saw in Chapter 2 how the pavement of a street show some polarization. So will lava! In fact, many dark materials tend to be polarized in this manner.
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The lava at the bottom of the Kiluea Iki crater in Hawaii appears brighter in the left picture than the right. In the left picture the light reflecting off the ground is partly polarized in the horizontal direction, which is the same direction as the transmission axis of the Polaroid sunglasses.
Because water produces horizontally polarized light upon reflection, wet and dry ground have different polarization characteristics. In the upper pair of pictures, the ground is dry. In the lower pair, it is wet.
Rainbows & Halos
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When light hits a raindrop, part of the light is reflected and part refracted. Both parts will be partly polarized, but in different directions. The same thing occurs when the refracted beam A hits the surface again (this time from the inside), etc.
It should not be surprising that rainbows are polarized! |
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A phase shift in one component of polarization of light in a raindrop causes a phase shift. This makes two supernumary bows, slightly shifted in position. |
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Ice Crystals
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Depending on how light hits it, the passage of light through ice crystals can be polarized as well. Thus, many phenomena such as parhelia (sundogs) and solar pillars will be affected.
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The picture on the left is produced by passing light through a quartz crystal. Because the index of refraction is slightly different for the two planes of polarization, two “sundog”-like) spots of light are produced, each with a different plane of polarization. This can be seen by placing a Polaroid filter in the beam, as was done in the lower half of the picture. This is not easily seen in real parhelia, as the two components are separated by only 1/7th the angular separation as for quartz.
Windows
Light reflecting off of or refracting into window glass will become partly polarized.
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A person may be difficult to see through glass that has skylight reflecting off of it. On the right, they can be seen more easily with the insertion of a properly-oriented Polaroid filter.
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Glass and metals have somewhat different properties. Glass tends to produce greater polarization when reflected, but in both cases the orientation is horizontal for a horizontal surface. |
Automobiles
The picture of the woman behind the glass further up the page was actually behind the windshield of a car.
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This is one reason why sunglasses, with their transmission axes oriented vertically, help reduce the glare of sunlight reflecting off of cars.
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Bodies of Water (more)
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Light reflecting off of water produces polarized glare, especially if it occurs at an angle greater than Brewster’s angle. The light that actually penetrates into the water will also be partly polarized vertically.
Polarization in Animals
Scientific studies have shown that the eyes of some animals are sensitive to the polarization state of light in a significant way.
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This is an electron micrograph of the structure in the cones of the eye of a crayfish. The orientation of these rod-like structures, horizontal to you in the top half, pointing toward you in the bottom half, act like little Polaroid filters! |
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The structure inside the eye of a spider crab is shown here. |
It appears that animals use the polarization of light for orientation.
For more on the phenomenon of polarization-sensitivity in animal eyes, visit this nice site:
http://polarization.com/index-net/index.html
Polarization OF Animals
Interestingly, many structures in living things have spiral structures. This can introduce circularly polarized light! An example is the back of this beetle, where left- and right- circularly polarized light are reflected differently.
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Finding polarized light in the environment can be fun as well as educational. Keep your filters and play with them!
