Developing Useless Superpowers 101: How to detect polarized light

Illustration for article titled Developing Useless Superpowers 101: How to detect polarized light

There is a subtle indicator that tells you whether or not you're seeing polarized light, and which direction the light is polarized on. Find out about Haidinger's Brush.

Polarization of light has come in handy over the last few years. Light travels in waves, like the kind you would see traveling along a suspended piece of string if you pulled one end rapidly up and down. Most naturally-emitted light waves are oriented every which way - the crests and valleys can be oriented up and down, side to side, or at any diagonal angle.

Polarized light, on the other hand, has all its waves oriented only one way. Light can be polarized when it reflects off something, refracts through something, or it can be emitted already polarized. One of the easiest ways to polarize light is just to make it go through slits oriented perpendicular to the direction that you'd like light to be polarized. All the light that's oriented in the wrong direction will be blocked as it tries to get through the slit. Many pairs of glasses are covered with a coating with tiny, imperceptible slits which do just that.

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Windshield and sunglass makers use polarization a lot, to make lenses that reduce glare. When light reflects off water or snow, or the horizontal surfaces of cars, much of it is polarized horizontally. The reflection of a lot of sun makes the surface look like a giant sheet of light which drowns out all the light around it. By making glass with a coating that polarizes light vertically, most of the horizontal glare is choked off, allowing the person behind the glass to see more clearly.

Movies that use 3D use polarization as well. They film the movie from two different angles, present one angle only in horizontally polarized light, and one in only vertically polarized light. By making the audience wear glasses, one lens of which only allows vertical light and the other only allows horizontal light, the movie presents two different images, which the viewer's brain interprets as one three dimensional image.

One of the advantages of polarization is human's lack of perception of it. Some animals can differentiate between light polarized in different directions, but for humans it's just a handy way of filtering out a certain percentage of the incoming light without changing the perceived image.

OR IS IT?

There is one way for humans to tell if they're not getting all the light they're entitled to, and you can train yourself to look for it. Unfortunately, learning the trick is a little like learning to fly in a Douglas Adams' novel. You can't concentrate on it, or think about it, because the moment your brain realizes it's not ‘reality', you can't do it anymore. It requires practice and just the right amount of focus.

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And it requires attentiveness. When polarized light hits the human eye, a figure called Haidinger's Brush appears. Discovered in the 1800s by Wilhelm Haidinger, the ‘brush' resembles a pair of crisscrossed bowties; one yellow, one blue. The blue bow runs parallel to the axis of light's polarization, and the yellow is perpendicular. The bows will rotate with the polarization, allowing you to know if it's shifting.

Illustration for article titled Developing Useless Superpowers 101: How to detect polarized light
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Some people say that red or blue glass will help you spot Haidinger's Brush, but most agree that your best bet is monochromatic light and polarizing filters, since that will help the colors pop. Polarized lenses from glasses and polarized film will help. Rapidly rotating a piece of paper in sunlight is a cheaper way to practice. Or you could go high tech and tilt your head back and forth while staring at a plain white image on an LCD computer screen. Whatever you do, remember that the image will fade when your brain realizes that it's not seeing anything useful. Looking away and then back, will help. In time, seeing Haidinger's Brush will be easier.

[Via James Calvert, Polarization.net, and World.]

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DISCUSSION

"Polarized light, on the other hand, has all its waves oriented only one way."

Not true.

Linearly-polarized light has its waves oriented only one way, but there are other forms of polarization, such as circular polarization. In circularly-polarized light, the light's electric field vector rotates as the wave propagates. See the animated gif below.

Circular polarization:

[upload.wikimedia.org]

"One of the easiest ways to polarize light is just to make it go through slits oriented the direction that you'd like light to be polarized."

Not true. For example, if you want to linearly-polarize a microwave beam along the vertical direction, you should make the beam pass a metal grid with horizontal slits.

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A little-known fact related to light polarization is the following. It turns out that if you look at the sky in a direction perpendicular to the line joining you and the sun (that is, the sun is to your left or right), the light reaching your eyes is strongly polarized along the vertical direction. On the other hand, looking towards the sun or away from it (the sun is in front of you or behind you), the light that reaches your eyes is unpolarized. You can test this yourself by using your sunglasses, assuming that they're the polarizer type.

This fact and the fact that certain kinds of crystals are natural polarizers allow you to determine the position of the sun even when you can't actually see it.

Legend has it that the Vikings (who lived near the Arctic circle) used this to aid in their navigation when the sun was just below the horizon.

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Another interesting example of polarization is that transparent materials change their optical properties under stress, causing light that passes through them to be polarized differently depending on their path through the material.

If you look at a transparent plastic ruler, for example, while wearing your polarizer sunglasses, you'll see different images when you twist the ruler. You'll see different colors too, since the refractive properties are also changed when the material is stressed.

This actually has some very important practical applications. When airplanes and other large and/or expensive objects are designed, small-scale versions made of transparent plastic are made, put under typical stresses, and looked upon through a set of polarizers. This allows the designers to literally see the points where the structure is at its weakest.