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You, too, can see which way light is polarized!

Friends-Romans-Countrymen, Math and Science, Outdoors

I was reading some articles about various cephalopods recently, as I often do, and one of them mentioned that some of the molluscs can “see” the polarization of light.  This was touted as a sense that humans do not have, and I thought that was very interesting.  However, it prompted me to do some searches about polarized light and I found out that some (most?) people can, in fact, see the polarization of light if they know what to look for.

For people reading this who have forgotten (or not taken) their high school science, let me provide this little primer…

Light (indeed all electromagnetic radiation including radio and X-rays) can be viewed as a wave with an electric field and a magnetic field at right angles to each other:

Light as an electromagnetic wave.
Light as an electromagnetic wave.

The electric field is often denoted with the symbol “E”, and the magnetic field with “B”.  The electric and magnetic fields are at right angles to each other, and as shown in the diagram, they have an orientation.  Polarization refers to the orientation of the plane electric field of a light wave.  In the figure, the wave is vertically polarized.

In normal light, that we see everywhere, the polarization of any given wave is essentially random.  That is to say, the light isn’t oriented in any particular plane.  However, a number of devices and effects can cause light to become more (or completely) oriented in a single plane (polarized).  Polarized sunglasses, for example, use treated glass/plastic lenses to pass light oriented only in a certain way.  Polarized sunglasses are useful because light reflected from non-metallic surfaces (like pavement or water) becomes polarized to some degree.  The sunglasses have their polarizing filters oriented in a way that blocks the usual polarization of reflected light.  Hence, polarized sunglasses cut “glare”… or if you are a fisherman, allow you to see into the water on a bright day.

We see our sky as blue due to the scattering of light from the sun by the air.  This scattering has the effect of polarizing the light in the sky and you can see this if you have polarizing sunglasses – take them off and look at the blue sky through them at arms length (DO NOT LOOK AT THE SUN, DUH). If you rotate the glasses, you are rotating the polarizing filter and you’ll see the sky brighten and darken as the polarization of the sunglasses matches and counters the polarization of the sky.

Now, if you’re looking at the sky, at an angle of about 90 degrees from the sun, you might also see something very faint that looks like this:

Haidinger's Brush - how the human eye sees polarized light.
Haidinger’s Brush – how the human eye sees polarized light.

An Austrian physicist, Wilhelm Haidinger saw that in 1844, and the effect became known as Haidinger’s Brush.  And what it is, is the visual representation that humans see when looking at polarized light.  The direction of polarization (the plane of the electric field) is perpendicular to the yellow bar (or in line with the blue bar, if you prefer).

Now, for my part, I haven’t seen it against the sky yet.  Haidinger’s brush is very faint, although I’m told it stand out against a blue background well.  There is, however, another place where you can see it.  LCD screens (not CRT) use polarizers as part of the display.  So if you are looking at an LCD monitor, you are getting a face-full of polarized light.  LCD screens are usually polarized diagonally, so when you see the brush, it will typically be tilted relative to the image above.

If you click these images, they should open to large-size, single colour fields of white and blue respectively.  Have a look and see if you can see them.  Personally, I see it best on the white background and if I tilt my head a bit.

white
white

sky blue
sky blue

blue
blue

So try it and see.  The effect is faint and small.  For me, on a 5760×1080 screen, the effect is about the size of the squares above.  The Wikipedia article is interesting too.

And there you go – a sense you probably didn’t even know you had!

Leave a comment if you can see it, and if you see it better on the white or the blue.  Try it in the sky too and let me know if you can see it there.

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